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Patrick Turner Patrick Turner is offline
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Default Gilbert Cell AM modulator and demodulator.

I'm having fun mucking around with 6 x PN100 garden variety
transistors used to make a discrete Gilbert Cell, like the bjts inside
an MC1496.
Instead of a rail of +12Vdc and two CCS bjts for the "bottom pair"
LTP, I have a +41Vdc rail and -30Vdc rail because the bjts are quite
happily biased with a max Vce = 15Vdc approx.

Anyway, I'm trying to produce an RF AM signal generator better than my
present one which uses a couple of triodes for an oscilator and CF
buffer, followed by a pentode RF amp which is grid modulated and with
Rk bypassed for RF only. The oscilator and pentode tank coils are
tuned with a two gang tuning cap from an old AM radio. Tracking is
good enough. At 90% audio modulation the envelope THD is about 5% 2H
and I want 97% mod with 1% max Dn, and without using loop FB from a
detector in the unit, or resorting to PP operation.

So I thought a Gilbert Cell might work better but with balanced or
unbalanced AF and RF inputs all I get is a DSB signal, ie, the two
side bands and no RF carrier.

I have not got as far as producing an AM detector using the Gilbert
cell yet.

Anyone else have any experience worth relating to the group?

Patrick Turner.
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Patrick Turner Patrick Turner is offline
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Default Gilbert Cell AM modulator and demodulator.

On Nov 13, 2:17*pm, "Alex" wrote:
"Patrick Turner" wrote in message

...





I'm having fun mucking around with 6 x PN100 garden variety
transistors used to make a discrete Gilbert Cell, like the bjts inside
an MC1496.
Instead of a rail of +12Vdc and two CCS bjts for the "bottom pair"
LTP, I have a +41Vdc rail and -30Vdc rail because the bjts are quite
happily biased with a max Vce = 15Vdc approx.


Anyway, I'm trying to produce an RF AM signal generator better than my
present one which uses a couple of triodes for an oscilator and CF
buffer, followed by a pentode RF amp which is grid modulated and with
Rk bypassed for RF only. The oscilator and pentode tank coils are
tuned with a two gang tuning cap from an old AM radio. Tracking is
good enough. At 90% audio modulation the envelope THD is about 5% 2H
and I want 97% mod with 1% max Dn, and without using loop FB from a
detector in the unit, or resorting to PP operation.


So I thought a Gilbert Cell might work better but with balanced or
unbalanced AF and RF inputs all I get is a DSB signal, ie, the two
side bands and no RF carrier.


I have not got as far as producing an AM detector using the Gilbert
cell yet.


Anyone else have any experience worth relating to the group?


Patrick Turner.


Alex:
The most linear modulator is a CMOS changeover (SPDT) switch, e.g., DG445,
DG442 or others with lower resistance. Get the switch fly with the carrier
frequency from some voltage to ground. Then the output will be perfectly
chopped between this voltage and zero. Thus the voltage becomes the envelope
of the output modulated signal. If this voltage is audio voltage -- here you
have a modulator. If the voltage has no DC, the output will be DSB, if you
offset it by DC component, you will have AM.


I have not tried the CMOS option.

But I only could get DSB because I had not understood the importance
of the slight dc bias difference needed made possible by the "carrier
null" adjustment. I made my circuit with all bases biased with 27k
taken to a bias voltage totem pole from the regulated PSU rail, and
all inputs are via 0.47uF coupling caps.

Bias R may need to be smaller and I am about to provide the necessary
bias imbalance across the bottom modulator LTP so I should be able to
either have DSB with equal biasing, or AM, with unequal, ie, be able
to "tune" the carrier out for DSB.

AF signals in the top LTP resistor collector loads could be suppressed
by following CR couping to the necessary buffer OP stages, or perhaps
a change to a CT RF choke from B+ rail to collectors with some
resistance damping load.



The voltage (signal) source
shall have low impedance at RF. Output needs to go through LPF since it is
basically rectangular and "dirty" for "broadcasting. Note that LPF shall
start with an inductor (not capacitor) for the same reason -- the signal is
rectangular.


We'll see.

You might be put off by the digital flavour of the thing.


Depends on ultimate performance.

The AM gene is only meant to make a blameless AM test signal; I am not
planning to use it for a broadcast station.
Presumably switching artifacts at above max carrier F at 1.7MHz could
be filtered out.

If this is the
case, here is another more analogue solution. Use not a double-balanced
modulator (which you call Gilbert cell), but simply a differential pair (2
bjts emitter coupled). Apply about 100...150mV of carrier between their
bases. This is easily done by putting one or two turns of tickle winding on
top of your tuce oscullator (which I guess has about 10...20V on the grid).


Yee-Ahh, the tube Osca Later puts out huge signal voltage compared to
****y little SS circuits.....:-) !!

I guess the few turns at the earthy end of the tube tank coil would be
fine, and can be CT, for balanced....


Now between the collectors the modulated signal will be almost exactly
proportional to the current in the long tail. E.g., if you make the current
swing from 1 to 20mA you will have 95% AM. Note that impedance of the long
tail shall be large, and capacitance as small as possible to maintain
linearity up to very small current. Thus it is the best to have a simple
resistor, say 1K directly going to the emitters. Another resistor in series
with the former will probably go to negative supply rail. Apply audio to the
junction of these two resistors.

Pick up the AM from the collectors via centre tapped winding. RF load
resistance should be low enough to avoid clipping. Do not forget
anti-parasitic 20...50R stoppers in the bases or collectors.

I would recommend to build cascodes on top of such modulator. This will
nullify base-collector capacitance and eliminate interplay between the
oscillator and antenna tanks. Thus your AM will be perfect -- without mix of
parasitic FM.


It sounds good this idea. I have some BF470/BF469 video transistors
with good ratings and perhaps can be used to replace the single 6BX6
pentode I am currently using. The tuned tank for SE operation would
also have to go.

There is a similarity between my 6BX6 modulator stage and your
suggestion which I have seen at a couple of sites I visited.

The 6BX6 is set up in pentode with screen bypassed to cathode and with
RF input fed via about 100pF to grid1.
The bias resistance to grid1 is taken to the AF source, so the AF
alters the bias current. The cathode resistance is greater than it
needs to be for a normal gain stage but cathode is bypassed to 0V by
sufficient C to form a low Z path so that RF gain is unaffected by Rk
and only affected by the change in gM of the pentode when the bias
current changes between a lot, and hardly anything, ie, enough to make
modulation from nil to 95% when Dn makes things begin to look ugly.

I keep the anode load high because the anode has a tuned LC circuit
with tap at one end near the B+ which then feeds the OP terminal for a
lowZ signal.

But first I want to get the Gilbert Cell working better, so I have
something to compare with other methods.


Using the GC for AM de-modulation means i'd have to make what is
effectively a synchronous detector.
There needs to be a carrier of fixed amplitude applied to the GC and
this is done by feeding the small signal AM modulated carrier or IF
signal to a limiting amp, ie, an amp driven into overdriven so it
clips. There must at least be some AM RF carrier present at the
limiter amp input even where AM % becomes very large and when the
modulated carrier present falls close to zero or else the limiting is
stopped and you get severe N&D. I remember finding the "locked
limiter" did not work perfectly, and was one reason I could not
proceed.

I guess a PLL oscillator which would "run on" during short time
absences of a synchronized and limited carrier would be better. The
last time I tried was 1999 when I spent a week trying t get a tubed
"Synchrodyne" first designed by a Mr Tucker in 1947, and published by
Wireless World that year. It never was a commercial success and could
not compete with good superhet performence, at least for broadcast AM
bands. Besides, stations today routinely use right up to 100%
modulation.

Anyway, my synchrodyne was very hard to get working anywhere near as
well as a good superhet even when I resorted to using a much simpller
circuit with 6EJ7 input RF gain amp and a 6BE6 self oscillating AM
detector; plus there's the howls and whistles when tuning and the the
monkey chatter from other stations.

Patrick Turner.



s

Regards,
Alex- Hide quoted text -

- Show quoted text -


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Alex Alex is offline
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Default Gilbert Cell AM modulator and demodulator.


"Patrick Turner" wrote in message
...
I'm having fun mucking around with 6 x PN100 garden variety
transistors used to make a discrete Gilbert Cell, like the bjts inside
an MC1496.
Instead of a rail of +12Vdc and two CCS bjts for the "bottom pair"
LTP, I have a +41Vdc rail and -30Vdc rail because the bjts are quite
happily biased with a max Vce = 15Vdc approx.

Anyway, I'm trying to produce an RF AM signal generator better than my
present one which uses a couple of triodes for an oscilator and CF
buffer, followed by a pentode RF amp which is grid modulated and with
Rk bypassed for RF only. The oscilator and pentode tank coils are
tuned with a two gang tuning cap from an old AM radio. Tracking is
good enough. At 90% audio modulation the envelope THD is about 5% 2H
and I want 97% mod with 1% max Dn, and without using loop FB from a
detector in the unit, or resorting to PP operation.

So I thought a Gilbert Cell might work better but with balanced or
unbalanced AF and RF inputs all I get is a DSB signal, ie, the two
side bands and no RF carrier.

I have not got as far as producing an AM detector using the Gilbert
cell yet.

Anyone else have any experience worth relating to the group?

Patrick Turner.


Alex:
The most linear modulator is a CMOS changeover (SPDT) switch, e.g., DG445,
DG442 or others with lower resistance. Get the switch fly with the carrier
frequency from some voltage to ground. Then the output will be perfectly
chopped between this voltage and zero. Thus the voltage becomes the envelope
of the output modulated signal. If this voltage is audio voltage -- here you
have a modulator. If the voltage has no DC, the output will be DSB, if you
offset it by DC component, you will have AM. The voltage (signal) source
shall have low impedance at RF. Output needs to go through LPF since it is
basically rectangular and "dirty" for "broadcasting. Note that LPF shall
start with an inductor (not capacitor) for the same reason -- the signal is
rectangular.

You might be put off by the digital flavour of the thing. If this is the
case, here is another more analogue solution. Use not a double-balanced
modulator (which you call Gilbert cell), but simply a differential pair (2
bjts emitter coupled). Apply about 100...150mV of carrier between their
bases. This is easily done by putting one or two turns of tickle winding on
top of your tuce oscullator (which I guess has about 10...20V on the grid).
Now between the collectors the modulated signal will be almost exactly
proportional to the current in the long tail. E.g., if you make the current
swing from 1 to 20mA you will have 95% AM. Note that impedance of the long
tail shall be large, and capacitance as small as possible to maintain
linearity up to very small current. Thus it is the best to have a simple
resistor, say 1K directly going to the emitters. Another resistor in series
with the former will probably go to negative supply rail. Apply audio to the
junction of these two resistors.

Pick up the AM from the collectors via centre tapped winding. RF load
resistance should be low enough to avoid clipping. Do not forget
anti-parasitic 20...50R stoppers in the bases or collectors.

I would recommend to build cascodes on top of such modulator. This will
nullify base-collector capacitance and eliminate interplay between the
oscillator and antenna tanks. Thus your AM will be perfect -- without mix of
parasitic FM.

Regards,
Alex


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Patrick Turner Patrick Turner is offline
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Default Gilbert Cell AM modulator and demodulator.

I can now add to what I said previously about making an AM modulator
for testing AM radios.

I set up a Gilbert Cell using 3 pairs of NPN bjts in 3 LTPs. it was
based on the Onsemi application notes for the MC1496 IC which s a
balanced modulator chip with a gilbert cell within.

Initially, because I had no "carrier null" adjustment pot all I could
make was a DSB signal.

I worked out a suitable way to adjust the biasing of the bottom LTP
for AF input so I could adjust the dc bias for a DC offset for a
slightly unbalanced DC condition. This made the circuit able to vary
its function beween producing carrier + AF modulating signal just
mixed, or a DSB signal or an AM signal. The AM was a bit fiddly to
adjust so the max carrier input was optimised for the max AF
modulation signal and carrier inclusion level. After adjusting the 3
adjustables, I could get fairly linear AM up to 100% of any carrier
signal of about 2Vrms output at each of the 2 OP ports. The single
carrier input needed to be about 0.1Vrms and single AF input needed
was 1.6Vrms for 2Vrms of output from each port. But at 2Vrms output
with unmodulated carrier there was 3H THD of about 5%, and the ability
to make 2V above 200kHz began to decline and by 1.6MHz the carrier
signal when unmodulated becomes highly distorted with odd + even H. So
in reality, there is a serious restriction on RF bandwidth and from
distortion.
I am using a a B+ rail of +41V to get the advantage of dynamic
headroom. In effect, one can only expect about 1Vrms of output carrier
voltage up to 300kHz if one wishes to keep the carrier pure. At below
100kHz, the modulation linearity looks very linear when the AF wave
input is overlaid on top of the envelope shape up to and beyond 100%
modulation.

But once above about 100kHz the envelope shape begins to become
distorted and by 455kHz there is about 5% Dn right up to the point of
100% modulation where the carrier level becomes zero on wave troughs.
The postive wave crests are taller than the wave troughs, so envelope
Dn becomes about 5% at 100% modulation. So the transistor modulator is
effectively no better than the SE pentode which I have been using so
far with its very ordinary grid modulation.

I will investigate the use of smaller/larger emitter resistors to see
if that widens the BW range and betters the linearity.

If the gilbert cell cannot be made t work as well as the pentode it
would not be too hard to make up a single LTP with two bjts with
balanced carrier input and with AF modulation signal applied to the
common emitter circuit.

There is a remarkably large pile of BS on AM moduation online. Hardly
any schematics exist except basic types with far worse operation than
I am trying for, and there is almost no reports on test results.

Patrick Turner.



On Nov 12, 3:21*pm, Patrick Turner wrote:
I'm having fun mucking around with 6 x PN100 garden variety
transistors used to make a discrete Gilbert Cell, like the bjts inside
an MC1496.
Instead of a rail of +12Vdc and two CCS bjts for the "bottom pair"
LTP, I have a +41Vdc rail and -30Vdc rail because the bjts are quite
happily biased with a max Vce = 15Vdc approx.

Anyway, I'm trying to produce an RF AM signal generator better than my
present one which uses a couple of triodes for an oscilator and CF
buffer, followed by a pentode RF amp which is grid modulated and with
Rk bypassed for RF only. The oscilator and pentode tank coils are
tuned with a two gang tuning cap from an old AM radio. Tracking is
good enough. At 90% audio modulation the envelope THD is about 5% 2H
and I want 97% mod with 1% max Dn, and without using loop FB from a
detector in the unit, or resorting to PP operation.

So I thought a Gilbert Cell might work better but with balanced or
unbalanced AF and RF inputs all I get is a DSB signal, ie, the two
side bands and no RF carrier.

I have not got as far as producing an AM detector using the Gilbert
cell yet.

Anyone else have any experience worth relating to the group?

Patrick Turner.


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Patrick Turner Patrick Turner is offline
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Posts: 3,964
Default Gilbert Cell AM modulator and demodulator.

On Nov 14, 11:28*pm, flipper wrote:
Well, RF is outside my realm but during various roamings I recall
seeing a couple of things that might give you something to at least
search for.

The first is I seem to recall comments about "pre-distortion" used to
compensate the Gilbert cell. Sorry that I can't recall more on that
topic.

The other is the Gilbert cell being very sensitive to component
mismatch. That makes sense as we all know a 'diff amp' is and with the
Gilbert being one feeding the emitters of two others one can only
imagine the problem being worse, especially with discretes.

Have you tried matching the transistors and some emitter degeneration?


Yes indeed I tried adding in emitter resistors to the top 4 bjts and
this worsened all operation, and raised the carrier distortion at 1/2
max Vo to about 4%, and the signal looked like a class B amp with 3H
crossover THD, YUK.

I could not get the circuit to give a clean reduction of envelope
shape to a straight line at the positive and negative maximum envelope
swings.

Anyway, I understand the Gilbert cell now, but I won't be using it as
an alternative to the tube signal gene with AM.

I then replaced the 6BX6 pentode with 6EJ7 in my SE AM modulator which
has a tuned RF transformer circuit for anode load.
The tuned anode coil is wrapped with a much smaller secondary coil
which is grounded one end and feeds aa 2k0 R divider load to vary the
output 0dB to -30dB in six switch positionslevel at the output
terminal. At -30dB Rout = 50 ohms and the low signal is strong enough
to drive all AM radios with say 5pF coupling to the antenna terminal.

The 6EJ7 only very slightly reduced the envelope THD, but showed it
could produce an over modulated signal with less AF drive.
I also tried a 6EH7 which is the variable µ or "remote cut off"
version of the 6EJ7, but envelope THD was much worse.

The SE pentode modulator was checked out for any faults in DC supplies
and basic condition, the first time in 13 years. No problems.

The pentode is set up with B+ = 250V and Eg2 = 225V via 33k and 0.1uF
to 0V.

The cathode has Rk = 1k8, and grid is biased with 1M to 0V. The RF
signal comes from a 12AT7 CF buffer after the 12AT7 Hartley oscillator
with a single tuned grid LC. When the RF ranges are is displayed on a
CRO, you cannot see any form of THD so I guess THD 0.5%
Levels are about 7Vrms. Easy peasy for 1/2 f a 12AT7.

The RF buffer driver has 0.02uF then 15k to pentode grid. Then I have
a 22k from pentode grid to a 0.1uF which couples to a 12AU7 CF which
buffers the audio input signal. The RF level before the modulator
remains fixed and determined by the oscillator level which does not
change much.

So in effect, I have RF between 360kHz and 1.75MHz in two ranges and
simply mixed with an audio signal in a linear resistance network.

But the pentode is set up to have no audio gain at its anode because
its load at AF is an RF coil of only 190uH, with tuning C max below
1,000pF.
At the the anode the LC is tuned to the same F as the oscillator
frequency, and the parallel LC has maybe 10k tuned impedance, so at RF
the pentode produces plenty of RF gain, but not excessive gain, and
the gain is slightly loaded by the secondary load of 2k0 resistance
divider load.
The LC RF transformer functions to keep any AF out of the anode output
but meanwhile, the high RF gain is possible because the Rk is bypassed
with 0.0033uF which has low enough Z at the range especially at high
RF, so that the slight roll off in RF level at HF is slightly
compensated.

The 0.0033uF is a high Z at below 20kHz so that there is an audio
signal just below the grid input audio signal appearing at the
cathode. Thus the 6EJ7 functions as a CF at AF. The grid input signal
is thus generating a fairly linear change of voltage across the Rk and
therefore a fairly linear change of anode current. There is also a
change of AF anode current due to the Vk to Vg2 change but its
effect is minor, and can be neglected.

The change of anode current at AF causes a corresponding change of
current in the tuned circuit and a fairly linear envelope shape. To
check the linearity, I measured more carefully before abandoning my
simple little modulator.

This time I used my low THD signal 0.01% from my 1kHz oscillator.
I could say that the 12AU7 buffer input to the modulator would
generate maybe 0.2% THD at the 7Vrms grid drive needed.

I set the CRO to monitor the peaks of the un-modulated carrier at the
centre horizontal gratical. and adjusted the modulation and CRO level
so that the maximum peak RF voltage with full modulation was twice the
peak RF voltage with no modulation. The only problem I could see was
the slight inability of the modulation to reduce to zero on wave
trough minimums.
Considering the positive or top side of the modulated carrier wave, So
I could get a +ve envelope Vswing of 2.0 times the positive swing of
the unmodutaed carrier, while the negative swing side was 1.90 x
unmodulated carrier.

The negative going envelope measured almost identical, and no worse.
From RDH, THD % = 100 x ( 0.5 [ 2.0 - 1.9 ] ) / ( 2.0 + 1.9 ) = 100
x (0.5 x 0.1) / 3.9 = 1.28%.

The THD rapidly reduced away from the maximum modulation. The Dn at
1.3% can be seen as a slight flat on the peaks of either +ve or -ve
sine wave shapes.

In a receiver with a simple diode + C + R detector of any kind, there
is a ripple voltage across the C, just like you get in a 1/2 wave PSU
rectifier. For the detector to function to filter the RF out and yet
allow the AF voltage to rise and fall linearly, the C must be just the
right value to give a time constant so that large high frequency AF
signals don't suffer slew distortion.
In my own detectors I use in most radios I use a CF buffer at the last
IF secondary and the cathode drives a germanium diode to charge a
220pF with 1M R which has 50Vdc idle voltage across it. This detector
of mine is extremely linear up until the carrier modulation reaches
over 90% and therefore begins to cut off the ripple voltage at the C
so that you get a flat on the wave trough at the instant of minumum
carrier level at high mod %. This happens at all AF, as a separate
effect to slew Dn.

The larger the R value in the diode RC network, the lower the Vripple,
and the less THD at high mod. If the R is low, and with bias current
from the DC across the R to keep the R current virtually constant, the
ripple can be too high, maybe a volt.
Then if IF signals are very low as they might be while listening to
short wave stations the whole detector functions with high distortion
Using a tube diode in a traditional circuit or in an "infinite
impedance detector" does not much change the outcome for SW listening.
The infinite Z detector in RDH4 does work fairly well, I tried that
recently, but the output is much below what my detector produces. Now
when the AF signal is over 2vrms at the C of the diode+C+R network the
Dn is very low except for the high peaks of mod % and as you should
know, not much signal on AM stations extends to over 90%. Those
signals that do manage 90% mod would often be heavily limited or
compressed peaks so the audio is already stuffed before it gets to a
radio detector.

The only way to reduce Dn due to ripple voltage cut off is with the
use of a NFB path + gain amp + diode+C+R, so that the slightest +v or -
ve going carrier cause the amp to put a larger voltage and current
into the C which then sends a FB signal back to the input to stop the
charging of the C during each 455kHz wave cycle. This means the C
voltage "follows the envelope" much more closely during the last 10%
of maximum modulation. Its usually done with an opamp, but could be
done with a shunt FB circuit around a power tube with an OPT which
operates at IF frequency. Although everyone says the opamp works best,
the tube should work fine and worth the trouble if the tube gain is at
least over 20.

To make sure one gets better SW reception which is usually pretty poor
in many AM radios, an RF input stage should be used to lift the SW
signals to a similar level produced by broadcast stations.

I am about to try a PP modulator with two transistors where un-
modulated RF is fed into a diff pair on one side, and the common
emitters are connected to a collector of a third bjt which has Re of
just the right value so that the AF changes at the iput base changes
the collector currents in both the diff pair bjts and in common mode.
I wound a coil for this which can be tuned to the carrier F. The diff
pair will have +41V collector supply with bases at +20V allowing a max
swing of 36V pk-pk. The RF coil load will keep AF out, but allow
differential RF and hopefully give less envelope distortion right up
to 100% mod and be a heck of a lot simpler than a Gilbert Cell.

Variable grid biasing which changes at a low AF rate can be used in a
PP OP stage of a guitar amp for where tremolo/vibrato is wanted. Its
something I have not yet explored well enough.

Patrick Turner.


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John Byrns John Byrns is offline
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Default Gilbert Cell AM modulator and demodulator.

In article ,
flipper wrote:

Well, RF is outside my realm but during various roamings I recall
seeing a couple of things that might give you something to at least
search for.

The first is I seem to recall comments about "pre-distortion" used to
compensate the Gilbert cell. Sorry that I can't recall more on that
topic.


It might be worthwhile to check one of the patent web sites on this,
IIRC the Motorola CQUAM guys patented a compensation circuit for the
MC1496 to make it work correctly in their early CQUAM exciters. The
same circuit might work with Patrick's discrete transistor gilbert cell
modulator, or I suppose it might not, being as I am not an expert in
this area.

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/
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John Byrns John Byrns is offline
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Posts: 1,441
Default Gilbert Cell AM modulator and demodulator.

In article
,
Patrick Turner wrote:

On Nov 14, 11:28*pm, flipper wrote:
Well, RF is outside my realm but during various roamings I recall
seeing a couple of things that might give you something to at least
search for.

The first is I seem to recall comments about "pre-distortion" used to
compensate the Gilbert cell. Sorry that I can't recall more on that
topic.

The other is the Gilbert cell being very sensitive to component
mismatch. That makes sense as we all know a 'diff amp' is and with the
Gilbert being one feeding the emitters of two others one can only
imagine the problem being worse, especially with discretes.

Have you tried matching the transistors and some emitter degeneration?


Yes indeed I tried adding in emitter resistors to the top 4 bjts and
this worsened all operation, and raised the carrier distortion at 1/2
max Vo to about 4%, and the signal looked like a class B amp with 3H
crossover THD, YUK.

I could not get the circuit to give a clean reduction of envelope
shape to a straight line at the positive and negative maximum envelope
swings.

Anyway, I understand the Gilbert cell now, but I won't be using it as
an alternative to the tube signal gene with AM.

I then replaced the 6BX6 pentode with 6EJ7 in my SE AM modulator which
has a tuned RF transformer circuit for anode load.
The tuned anode coil is wrapped with a much smaller secondary coil
which is grounded one end and feeds aa 2k0 R divider load to vary the
output 0dB to -30dB in six switch positionslevel at the output
terminal. At -30dB Rout = 50 ohms and the low signal is strong enough
to drive all AM radios with say 5pF coupling to the antenna terminal.

The 6EJ7 only very slightly reduced the envelope THD, but showed it
could produce an over modulated signal with less AF drive.
I also tried a 6EH7 which is the variable µ or "remote cut off"
version of the 6EJ7, but envelope THD was much worse.

The SE pentode modulator was checked out for any faults in DC supplies
and basic condition, the first time in 13 years. No problems.

The pentode is set up with B+ = 250V and Eg2 = 225V via 33k and 0.1uF
to 0V.

The cathode has Rk = 1k8, and grid is biased with 1M to 0V. The RF
signal comes from a 12AT7 CF buffer after the 12AT7 Hartley oscillator
with a single tuned grid LC. When the RF ranges are is displayed on a
CRO, you cannot see any form of THD so I guess THD 0.5%
Levels are about 7Vrms. Easy peasy for 1/2 f a 12AT7.

The RF buffer driver has 0.02uF then 15k to pentode grid. Then I have
a 22k from pentode grid to a 0.1uF which couples to a 12AU7 CF which
buffers the audio input signal. The RF level before the modulator
remains fixed and determined by the oscillator level which does not
change much.

So in effect, I have RF between 360kHz and 1.75MHz in two ranges and
simply mixed with an audio signal in a linear resistance network.


Hi Patrick,

How do you produce AM by simply mixing RF and audio in a linear
resistance network? This would be easier to understand if you had a
schematic available, is there one on your web page?

But the pentode is set up to have no audio gain at its anode because
its load at AF is an RF coil of only 190uH, with tuning C max below
1,000pF.
At the the anode the LC is tuned to the same F as the oscillator
frequency, and the parallel LC has maybe 10k tuned impedance, so at RF
the pentode produces plenty of RF gain, but not excessive gain, and
the gain is slightly loaded by the secondary load of 2k0 resistance
divider load.
The LC RF transformer functions to keep any AF out of the anode output
but meanwhile, the high RF gain is possible because the Rk is bypassed
with 0.0033uF which has low enough Z at the range especially at high
RF, so that the slight roll off in RF level at HF is slightly
compensated.

The 0.0033uF is a high Z at below 20kHz so that there is an audio
signal just below the grid input audio signal appearing at the
cathode. Thus the 6EJ7 functions as a CF at AF. The grid input signal
is thus generating a fairly linear change of voltage across the Rk and
therefore a fairly linear change of anode current. There is also a
change of AF anode current due to the Vk to Vg2 change but its
effect is minor, and can be neglected.

The change of anode current at AF causes a corresponding change of
current in the tuned circuit and a fairly linear envelope shape. To
check the linearity, I measured more carefully before abandoning my
simple little modulator.

This time I used my low THD signal 0.01% from my 1kHz oscillator.
I could say that the 12AU7 buffer input to the modulator would
generate maybe 0.2% THD at the 7Vrms grid drive needed.

I set the CRO to monitor the peaks of the un-modulated carrier at the
centre horizontal gratical. and adjusted the modulation and CRO level
so that the maximum peak RF voltage with full modulation was twice the
peak RF voltage with no modulation. The only problem I could see was
the slight inability of the modulation to reduce to zero on wave
trough minimums.
Considering the positive or top side of the modulated carrier wave, So
I could get a +ve envelope Vswing of 2.0 times the positive swing of
the unmodutaed carrier, while the negative swing side was 1.90 x
unmodulated carrier.

The negative going envelope measured almost identical, and no worse.
From RDH, THD % = 100 x ( 0.5 [ 2.0 - 1.9 ] ) / ( 2.0 + 1.9 ) = 100
x (0.5 x 0.1) / 3.9 = 1.28%.

The THD rapidly reduced away from the maximum modulation. The Dn at
1.3% can be seen as a slight flat on the peaks of either +ve or -ve
sine wave shapes.

In a receiver with a simple diode + C + R detector of any kind, there
is a ripple voltage across the C, just like you get in a 1/2 wave PSU
rectifier. For the detector to function to filter the RF out and yet
allow the AF voltage to rise and fall linearly, the C must be just the
right value to give a time constant so that large high frequency AF
signals don't suffer slew distortion.
In my own detectors I use in most radios I use a CF buffer at the last
IF secondary and the cathode drives a germanium diode to charge a
220pF with 1M R which has 50Vdc idle voltage across it. This detector
of mine is extremely linear up until the carrier modulation reaches
over 90% and therefore begins to cut off the ripple voltage at the C
so that you get a flat on the wave trough at the instant of minumum
carrier level at high mod %. This happens at all AF, as a separate
effect to slew Dn.

The larger the R value in the diode RC network, the lower the Vripple,
and the less THD at high mod. If the R is low, and with bias current
from the DC across the R to keep the R current virtually constant, the
ripple can be too high, maybe a volt.
Then if IF signals are very low as they might be while listening to
short wave stations the whole detector functions with high distortion
Using a tube diode in a traditional circuit or in an "infinite
impedance detector" does not much change the outcome for SW listening.
The infinite Z detector in RDH4 does work fairly well, I tried that
recently, but the output is much below what my detector produces. Now
when the AF signal is over 2vrms at the C of the diode+C+R network the
Dn is very low except for the high peaks of mod % and as you should
know, not much signal on AM stations extends to over 90%. Those
signals that do manage 90% mod would often be heavily limited or
compressed peaks so the audio is already stuffed before it gets to a
radio detector.

The only way to reduce Dn due to ripple voltage cut off is with the
use of a NFB path + gain amp + diode+C+R, so that the slightest +v or -
ve going carrier cause the amp to put a larger voltage and current
into the C which then sends a FB signal back to the input to stop the
charging of the C during each 455kHz wave cycle. This means the C
voltage "follows the envelope" much more closely during the last 10%
of maximum modulation. Its usually done with an opamp, but could be
done with a shunt FB circuit around a power tube with an OPT which
operates at IF frequency. Although everyone says the opamp works best,
the tube should work fine and worth the trouble if the tube gain is at
least over 20.

To make sure one gets better SW reception which is usually pretty poor
in many AM radios, an RF input stage should be used to lift the SW
signals to a similar level produced by broadcast stations.

I am about to try a PP modulator with two transistors where un-
modulated RF is fed into a diff pair on one side, and the common
emitters are connected to a collector of a third bjt which has Re of
just the right value so that the AF changes at the iput base changes
the collector currents in both the diff pair bjts and in common mode.
I wound a coil for this which can be tuned to the carrier F. The diff
pair will have +41V collector supply with bases at +20V allowing a max
swing of 36V pk-pk. The RF coil load will keep AF out, but allow
differential RF and hopefully give less envelope distortion right up
to 100% mod and be a heck of a lot simpler than a Gilbert Cell.

Variable grid biasing which changes at a low AF rate can be used in a
PP OP stage of a guitar amp for where tremolo/vibrato is wanted. Its
something I have not yet explored well enough.

Patrick Turner.


--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/
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Barry[_4_] Barry[_4_] is offline
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Posts: 9
Default Gilbert Cell AM modulator and demodulator.

"flipper" wrote in message
...
On Mon, 15 Nov 2010 03:31:56 -0800 (PST), Patrick Turner
wrote:

On Nov 14, 11:28 pm, flipper wrote:
Well, RF is outside my realm but during various roamings I recall
seeing a couple of things that might give you something to at least
search for.

The first is I seem to recall comments about "pre-distortion" used to
compensate the Gilbert cell. Sorry that I can't recall more on that
topic.

The other is the Gilbert cell being very sensitive to component
mismatch. That makes sense as we all know a 'diff amp' is and with
the
Gilbert being one feeding the emitters of two others one can only
imagine the problem being worse, especially with discretes.

Have you tried matching the transistors and some emitter
degeneration?


Yes indeed I tried adding in emitter resistors to the top 4 bjts and
this worsened all operation, and raised the carrier distortion at 1/2
max Vo to about 4%, and the signal looked like a class B amp with 3H
crossover THD, YUK.


Emitter degeneration lowers gain, of course.

Sorry, can't help much but I suspect it's discrete mismatch since I
gather IC versions do better than you're seeing.


I could not get the circuit to give a clean reduction of envelope
shape to a straight line at the positive and negative maximum envelope
swings.

Anyway, I understand the Gilbert cell now, but I won't be using it as
an alternative to the tube signal gene with AM.

I then replaced the 6BX6 pentode with 6EJ7 in my SE AM modulator which
has a tuned RF transformer circuit for anode load.
The tuned anode coil is wrapped with a much smaller secondary coil
which is grounded one end and feeds aa 2k0 R divider load to vary the
output 0dB to -30dB in six switch positionslevel at the output
terminal. At -30dB Rout = 50 ohms and the low signal is strong enough
to drive all AM radios with say 5pF coupling to the antenna terminal.

The 6EJ7 only very slightly reduced the envelope THD, but showed it
could produce an over modulated signal with less AF drive.
I also tried a 6EH7 which is the variable µ or "remote cut off"
version of the 6EJ7, but envelope THD was much worse.

The SE pentode modulator was checked out for any faults in DC supplies
and basic condition, the first time in 13 years. No problems.

The pentode is set up with B+ = 250V and Eg2 = 225V via 33k and 0.1uF
to 0V.

The cathode has Rk = 1k8, and grid is biased with 1M to 0V. The RF
signal comes from a 12AT7 CF buffer after the 12AT7 Hartley oscillator
with a single tuned grid LC. When the RF ranges are is displayed on a
CRO, you cannot see any form of THD so I guess THD 0.5%
Levels are about 7Vrms. Easy peasy for 1/2 f a 12AT7.

The RF buffer driver has 0.02uF then 15k to pentode grid. Then I have
a 22k from pentode grid to a 0.1uF which couples to a 12AU7 CF which
buffers the audio input signal. The RF level before the modulator
remains fixed and determined by the oscillator level which does not
change much.

So in effect, I have RF between 360kHz and 1.75MHz in two ranges and
simply mixed with an audio signal in a linear resistance network.

But the pentode is set up to have no audio gain at its anode because
its load at AF is an RF coil of only 190uH, with tuning C max below
1,000pF.
At the the anode the LC is tuned to the same F as the oscillator
frequency, and the parallel LC has maybe 10k tuned impedance, so at RF
the pentode produces plenty of RF gain, but not excessive gain, and
the gain is slightly loaded by the secondary load of 2k0 resistance
divider load.
The LC RF transformer functions to keep any AF out of the anode output
but meanwhile, the high RF gain is possible because the Rk is bypassed
with 0.0033uF which has low enough Z at the range especially at high
RF, so that the slight roll off in RF level at HF is slightly
compensated.

The 0.0033uF is a high Z at below 20kHz so that there is an audio
signal just below the grid input audio signal appearing at the
cathode. Thus the 6EJ7 functions as a CF at AF. The grid input signal
is thus generating a fairly linear change of voltage across the Rk and
therefore a fairly linear change of anode current. There is also a
change of AF anode current due to the Vk to Vg2 change but its
effect is minor, and can be neglected.

The change of anode current at AF causes a corresponding change of
current in the tuned circuit and a fairly linear envelope shape. To
check the linearity, I measured more carefully before abandoning my
simple little modulator.

This time I used my low THD signal 0.01% from my 1kHz oscillator.
I could say that the 12AU7 buffer input to the modulator would
generate maybe 0.2% THD at the 7Vrms grid drive needed.

I set the CRO to monitor the peaks of the un-modulated carrier at the
centre horizontal gratical. and adjusted the modulation and CRO level
so that the maximum peak RF voltage with full modulation was twice the
peak RF voltage with no modulation. The only problem I could see was
the slight inability of the modulation to reduce to zero on wave
trough minimums.
Considering the positive or top side of the modulated carrier wave, So
I could get a +ve envelope Vswing of 2.0 times the positive swing of
the unmodutaed carrier, while the negative swing side was 1.90 x
unmodulated carrier.

The negative going envelope measured almost identical, and no worse.
From RDH, THD % = 100 x ( 0.5 [ 2.0 - 1.9 ] ) / ( 2.0 + 1.9 ) = 100
x (0.5 x 0.1) / 3.9 = 1.28%.

The THD rapidly reduced away from the maximum modulation. The Dn at
1.3% can be seen as a slight flat on the peaks of either +ve or -ve
sine wave shapes.

In a receiver with a simple diode + C + R detector of any kind, there
is a ripple voltage across the C, just like you get in a 1/2 wave PSU
rectifier. For the detector to function to filter the RF out and yet
allow the AF voltage to rise and fall linearly, the C must be just the
right value to give a time constant so that large high frequency AF
signals don't suffer slew distortion.
In my own detectors I use in most radios I use a CF buffer at the last
IF secondary and the cathode drives a germanium diode to charge a
220pF with 1M R which has 50Vdc idle voltage across it. This detector
of mine is extremely linear up until the carrier modulation reaches
over 90% and therefore begins to cut off the ripple voltage at the C
so that you get a flat on the wave trough at the instant of minumum
carrier level at high mod %. This happens at all AF, as a separate
effect to slew Dn.

The larger the R value in the diode RC network, the lower the Vripple,
and the less THD at high mod. If the R is low, and with bias current
from the DC across the R to keep the R current virtually constant, the
ripple can be too high, maybe a volt.
Then if IF signals are very low as they might be while listening to
short wave stations the whole detector functions with high distortion
Using a tube diode in a traditional circuit or in an "infinite
impedance detector" does not much change the outcome for SW listening.
The infinite Z detector in RDH4 does work fairly well, I tried that
recently, but the output is much below what my detector produces. Now
when the AF signal is over 2vrms at the C of the diode+C+R network the
Dn is very low except for the high peaks of mod % and as you should
know, not much signal on AM stations extends to over 90%. Those
signals that do manage 90% mod would often be heavily limited or
compressed peaks so the audio is already stuffed before it gets to a
radio detector.

The only way to reduce Dn due to ripple voltage cut off is with the
use of a NFB path + gain amp + diode+C+R, so that the slightest +v or -
ve going carrier cause the amp to put a larger voltage and current
into the C which then sends a FB signal back to the input to stop the
charging of the C during each 455kHz wave cycle. This means the C
voltage "follows the envelope" much more closely during the last 10%
of maximum modulation. Its usually done with an opamp, but could be
done with a shunt FB circuit around a power tube with an OPT which
operates at IF frequency. Although everyone says the opamp works best,
the tube should work fine and worth the trouble if the tube gain is at
least over 20.

To make sure one gets better SW reception which is usually pretty poor
in many AM radios, an RF input stage should be used to lift the SW
signals to a similar level produced by broadcast stations.

I am about to try a PP modulator with two transistors where un-
modulated RF is fed into a diff pair on one side, and the common
emitters are connected to a collector of a third bjt which has Re of
just the right value so that the AF changes at the iput base changes
the collector currents in both the diff pair bjts and in common mode.
I wound a coil for this which can be tuned to the carrier F. The diff
pair will have +41V collector supply with bases at +20V allowing a max
swing of 36V pk-pk. The RF coil load will keep AF out, but allow
differential RF and hopefully give less envelope distortion right up
to 100% mod and be a heck of a lot simpler than a Gilbert Cell.

Variable grid biasing which changes at a low AF rate can be used in a
PP OP stage of a guitar amp for where tremolo/vibrato is wanted. Its
something I have not yet explored well enough.


Why not try beam deflection tubes, like the 6EM8? It'll do the whole
ball of wax, so to speak: Self oscillate and balanced modulator for
not only AM but SSB as well. They were, as the saying goes "made for
it." I hear they're quite linear and the datasheet looks linear,
except for the extremes but call that 'built in peak
limiting/compression' and a feature rather than 'distortion.

Put an AF amp on the front and an RF amp on the output and, voile, you
got an AM (or SSB) transmitter.

I keep wanting to use them for 'something' just because I think
they're cool but, as I mentioned, I'm not an RF guy.

My current 'fun project' is making a glow for show line level meter
with some UM80s I found for cheap and I just happen to have some 36VCT
2 buck power trannies that should be fine for filament and, with a
simple doubler, 100VDC B+. Doing a little transistor preamp to get the
signals right for the UM80.

If things work out I'll mount it in a cheap little wood 'jewelry' box
thingie I picked up for free.



You can get double sideband from a single balanced modulator, but it
would take a very sharp bandpass filter or a second balanced modulator
with proper phase shift networks to get SSB.

Probably the simplest circuit uses nothing more than a dual control
pentode such as the 6AS6/5725 or a dual control heptode such as the 6CS6.
The tube is configured as a conventional electron coupled oscillator, and
the suppressor is driven with a small audio source. If you have a
university library nearby, check out the article listed below. As long
as the suppressor grid is not internally connected to the cathode, an
ordinary pentode can be used. In these tubes, the suppressor is designed
to provide more control than in normal pentodes.

73, Dr. Barry L. Ornitz WA4VZQ

Green, C. B.: "Suppressor-grid Modulation," Bell Laboratory Record,
Vol. 17, pp. 41-44, October, 1938.


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Patrick Turner Patrick Turner is offline
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Default Gilbert Cell AM modulator and demodulator.

On Nov 16, 3:46*am, John Byrns wrote:
In article
,
*Patrick Turner wrote:





On Nov 14, 11:28*pm, flipper wrote:
Well, RF is outside my realm but during various roamings I recall
seeing a couple of things that might give you something to at least
search for.


The first is I seem to recall comments about "pre-distortion" used to
compensate the Gilbert cell. Sorry that I can't recall more on that
topic.


The other is the Gilbert cell being very sensitive to component
mismatch. That makes sense as we all know a 'diff amp' is and with the
Gilbert being one feeding the emitters of two others one can only
imagine the problem being worse, especially with discretes.


Have you tried matching the transistors and some emitter degeneration?


Yes indeed I tried adding in emitter resistors to the top 4 bjts and
this worsened all operation, and raised the carrier distortion at 1/2
max Vo to about 4%, and the signal looked like a class B amp with 3H
crossover THD, YUK.


I could not get the circuit to give a clean reduction of envelope
shape to a straight line at the positive and negative maximum envelope
swings.


Anyway, I understand the Gilbert cell now, but I won't be using it as
an alternative to the tube signal gene with AM.


I then replaced the 6BX6 pentode with 6EJ7 in my SE AM modulator which
has a tuned RF transformer circuit for anode load.
The tuned anode coil is wrapped with a much smaller secondary coil
which is grounded one end and feeds aa 2k0 R divider load to vary the
output 0dB to -30dB in six switch positionslevel at the output
terminal. At -30dB Rout = 50 ohms and the low signal is strong enough
to drive all AM radios with say 5pF coupling to the antenna terminal.


The 6EJ7 only very slightly reduced the envelope THD, but showed it
could produce an over modulated signal with less AF drive.
I also tried a 6EH7 which is the variable µ or "remote cut off"
version of the 6EJ7, but envelope THD was much worse.


The SE pentode modulator was checked out for any faults in DC supplies
and basic condition, the first time in 13 years. No problems.


The pentode is set up with B+ = 250V and Eg2 = 225V via 33k and 0.1uF
to 0V.


The cathode has Rk *= 1k8, and grid is biased with 1M to 0V. The RF
signal comes from a 12AT7 CF buffer after the 12AT7 Hartley oscillator
with a single tuned grid LC. *When the RF ranges are is displayed on a
CRO, you cannot see any form of THD so I guess THD 0.5%
Levels are about 7Vrms. Easy peasy for 1/2 f a 12AT7.


The RF buffer driver has 0.02uF then 15k to pentode grid. Then I have
a 22k from pentode grid to a 0.1uF which couples to a 12AU7 CF which
buffers the audio input signal. The RF level before the modulator
remains fixed and determined by the oscillator level which does not
change much.


So in effect, I have RF between 360kHz and 1.75MHz in two ranges and
simply mixed with an audio signal in a linear resistance network.


Hi Patrick,

How do you produce AM by simply mixing RF and audio in a linear
resistance network? *This would be easier to understand if you had a
schematic available, is there one on your web page?


My website is mainly devoted to audio matters and does not have the
schematics of the test gear I have built for myself.

I'll try to explain again.

There is a simple mix of RF voltage and AF voltage applied to an SE
pentode ( 6BX6/6EJ7), biased for normal SE class A operation.

The RF signal makes the pentode amplifiy it across a tuned tank in the
anode circuit.
The AF signal alters the grid biasing of the pentode thus changing the
gm of the tube between being twice the idle value and being almost nil
at cut off. The voltage gain of a pentode = gm x RL. The RF signal
current is modulated by the change in tube gm that is caused by the AF
change.

Its rather like a marriage. Two very different and independant ppl go
into a church to marry. Over the years one ends up modulating the
other, and they lose their originality to become a combined entity of
containing the original but now including the sum and differences of
themselves. If the noise and distortion of such a union is acceptable
and thus tolerable, they may remain together and radiate across the
universe to give a message. Sometimes someone else picks up the
message and casts away one of the partners, usually Mrs Arr Eff.

Patrick Turner.
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Big Bad Bob Big Bad Bob is offline
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Posts: 366
Default Gilbert Cell AM modulator and demodulator.

On 11/11/10 20:21, Patrick Turner so witilly quipped:
So I thought a Gilbert Cell might work better but with balanced or
unbalanced AF and RF inputs all I get is a DSB signal, ie, the two
side bands and no RF carrier.


This is absolutely correct. You've essentially built a 'ring
modulator'. If you want to run it like an SSB transceiver, you can then
filter the desired sideband before sending it to the PA. For AM, the
actual solution is simple: add the carrier back in with a subsequent
mixer circuit. Many SSB transcievers have a 'carrier' knob for this
purpose, for those cases where you can add carrier to give your signal a
bit more boost on reception.

I have not got as far as producing an AM detector using the Gilbert
cell yet.


Same thing as modulation, except it's carrier x 'modulated + carrier'
instead of carrier x modulation. It works best if the combined signal
is first sent through a crystal filter, then ultimately fed into a phase
locked loop oscillator with a slow time constant to provide the
'carrier'. This more or less means that you design your radio to have
crystal filters in the IF in lieu of transformers (at least for the
first and last IF stages).

This reminds me of ancient U.S. Navy communications gear, all tubes with
crystal ovens and complex synthesis circuits (they used semiconductor
diodes in the synth circuits, though, to generate harmonics).



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Big Bad Bob Big Bad Bob is offline
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Posts: 366
Default Gilbert Cell AM modulator and demodulator.

On 11/15/10 19:26, Patrick Turner so witilly quipped:
There is a simple mix of RF voltage and AF voltage applied to an SE
pentode ( 6BX6/6EJ7), biased for normal SE class A operation.

The RF signal makes the pentode amplifiy it across a tuned tank in the
anode circuit.
The AF signal alters the grid biasing of the pentode thus changing the
gm of the tube between being twice the idle value and being almost nil
at cut off. The voltage gain of a pentode = gm x RL. The RF signal
current is modulated by the change in tube gm that is caused by the AF
change.


if you're not ACTUALLY using a pentagrid tube as oscillator + modulator
you might get away with re-mixing phase-inverted carrier back with the
modulated signal, giving you sum + diff + MUCH LOWER CARRIER. Using a
spectrum analyzer you could then determine when you have 100%
modulation, and the THD would be MUCH LOWER since you would be able to
restrict operation to a more linear part of the modulation tube's gain
curves. You would need to have some pretty well aligned gear to make
this work, but with an AGC circuit built into the carrier oscillator (so
the output is very consistent) and careful design of the modulator, you
could get away with it. You'd effectively have a hi-fi AM modulator,
made with 3 or 4 RF tubes and a handful of additional components.

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Big Bad Bob Big Bad Bob is offline
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Default Gilbert Cell AM modulator and demodulator.

On 11/16/10 15:41, flipper so witilly quipped:
On Tue, 16 Nov 2010 15:27:29 -0800, Big Bad Bob
wrote:

On 11/11/10 20:21, Patrick Turner so witilly quipped:
So I thought a Gilbert Cell might work better but with balanced or
unbalanced AF and RF inputs all I get is a DSB signal, ie, the two
side bands and no RF carrier.


This is absolutely correct. You've essentially built a 'ring
modulator'. If you want to run it like an SSB transceiver, you can then
filter the desired sideband before sending it to the PA. For AM, the
actual solution is simple: add the carrier back in with a subsequent
mixer circuit. Many SSB transcievers have a 'carrier' knob for this
purpose, for those cases where you can add carrier to give your signal a
bit more boost on reception.


Balanced output is DSB but single ended would give you AM, wouldn't
it?


hmm.... yeah it might contain remnants of both the original and carrier.
Normally I'd just put a tuned circuit transformer across the
differential output and drive the next stage with the secondary, but I
suppose you could capacitor couple it instead and get a combined signal.
I'm not sure what levels everything would be at, though. Due to its
unpredictable nature, I'd say "better use the differential output and
adjust carrier level later". You'd get more predictable results that way.

Then again, creative engineers get paid a LOT of money to strip
'unnecessary' components out of a design during a cost-reduction phase.
If you could make it work with fewer components, go for it.

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Big Bad Bob Big Bad Bob is offline
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Default Gilbert Cell AM modulator and demodulator.

On 11/16/10 15:51, flipper so witilly quipped:
On Tue, 16 Nov 2010 15:38:50 -0800, Big Bad Bob
wrote:

On 11/15/10 19:26, Patrick Turner so witilly quipped:
There is a simple mix of RF voltage and AF voltage applied to an SE
pentode ( 6BX6/6EJ7), biased for normal SE class A operation.

The RF signal makes the pentode amplifiy it across a tuned tank in the
anode circuit.
The AF signal alters the grid biasing of the pentode thus changing the
gm of the tube between being twice the idle value and being almost nil
at cut off. The voltage gain of a pentode = gm x RL. The RF signal
current is modulated by the change in tube gm that is caused by the AF
change.


if you're not ACTUALLY using a pentagrid tube as oscillator + modulator
you might get away with re-mixing phase-inverted carrier back with the
modulated signal, giving you sum + diff + MUCH LOWER CARRIER. Using a
spectrum analyzer you could then determine when you have 100%
modulation, and the THD would be MUCH LOWER since you would be able to
restrict operation to a more linear part of the modulation tube's gain
curves. You would need to have some pretty well aligned gear to make
this work, but with an AGC circuit built into the carrier oscillator (so
the output is very consistent) and careful design of the modulator, you
could get away with it. You'd effectively have a hi-fi AM modulator,
made with 3 or 4 RF tubes and a handful of additional components.


So, if I understand, you're suggesting to limit initial modulation
depth to a low distortion range, say 75%, and then subtract 25%
carrier so the result appears 100%.

Did I get the idea right?


you got it. Then amplify the result to whatever level you need with a
subsequent stage, and/or use a voltage follower to drive a low Z load
directly.

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John Byrns John Byrns is offline
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Default Gilbert Cell AM modulator and demodulator.

In article ,
flipper wrote:

On Tue, 16 Nov 2010 16:12:41 -0800, Big Bad Bob
wrote:

On 11/16/10 15:41, flipper so witilly quipped:
On Tue, 16 Nov 2010 15:27:29 -0800, Big Bad Bob
wrote:

On 11/11/10 20:21, Patrick Turner so witilly quipped:
So I thought a Gilbert Cell might work better but with balanced or
unbalanced AF and RF inputs all I get is a DSB signal, ie, the two
side bands and no RF carrier.

This is absolutely correct. You've essentially built a 'ring
modulator'. If you want to run it like an SSB transceiver, you can then
filter the desired sideband before sending it to the PA. For AM, the
actual solution is simple: add the carrier back in with a subsequent
mixer circuit. Many SSB transcievers have a 'carrier' knob for this
purpose, for those cases where you can add carrier to give your signal a
bit more boost on reception.

Balanced output is DSB but single ended would give you AM, wouldn't
it?


hmm.... yeah it might contain remnants of both the original and carrier.


Well my understanding, granted limited, is the 'balanced' output is
two identical, but phase opposite, signals so the carrier nulls when
they're added and if that's correct then it seems to me like 'one' (or
the other) is simply AM.


It has been many many years since I looked at the Gilbert Cell, and I
don't have it in front of me now, however IIRC each of the "balanced"
outputs is the sum of two oppositely phased and modulated transistors,
so that the "carrier" is canceled even when you don't use both of the
balanced outputs. IIRC to get AM out, even from just one output, you
must add DC to the modulating audio signal.

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/
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Default Gilbert Cell AM modulator and demodulator.

On Nov 17, 10:27*am, Big Bad Bob BigBadBob-at-mrp3-
wrote:
On 11/11/10 20:21, Patrick Turner so witilly quipped:

So I thought a Gilbert Cell might work better but with balanced or
unbalanced AF and RF inputs all I get is a DSB signal, ie, the two
side bands and no RF carrier.


This is absolutely correct. *You've essentially built a 'ring
modulator'. *If you want to run it like an SSB transceiver, you can then
filter the desired sideband before sending it to the PA. *For AM, the
actual solution is simple: *add the carrier back in with a subsequent
mixer circuit. *Many SSB transcievers have a 'carrier' knob for this
purpose, for those cases where you can add carrier to give your signal a
bit more boost on reception.


When I got the Gilbert circuit to work fairly well with discretes, I
found I could adjust the 'carier null' pot to produce a signal with
just a simple resistance divider mix of RF carrier and AF. Then swing
to no DSB, but no carrier, then to AM.

Maybe using a 1496 chip would work better than 6 discretes. But I
learnt about this thing a bit.


I have not got as far as producing an AM detector using the Gilbert
cell yet.


Same thing as modulation, except it's carrier x 'modulated + carrier'
instead of carrier x modulation. *It works best if the combined signal
is first sent through a crystal filter, then ultimately fed into a phase
locked loop oscillator with a slow time constant to provide the
'carrier'. *This more or less means that you design your radio to have
crystal filters in the IF in lieu of transformers (at least for the
first and last IF stages).


I thought using a limiting amp locked to any incomming carrier would
provide an "exalted carrier" and thus when fed to a diode detector
there would be no flattening on weak waves when 100% mod occurs. The
PLL should indeed be one way if the locking IF signal is so low its
hard to amplify it to limiting, or noise ends up being amplified
instead.

But for FM audio detection, there is a 19kc pilot which is always
present and of sufficient amplitude that it can be fed into a second
input of an oscillator to phase lock the oscillator so that the 38kHz
result after a doubler does the business of converting DSB to being
easily detected with diode & CR. But far more thought and application
has preceeded me and I don't have time for everything, or to re-invent
the wheel.

This reminds me of ancient U.S. Navy communications gear, all tubes with
crystal ovens and complex synthesis circuits (they used semiconductor
diodes in the synth circuits, though, to generate harmonics).


Must of been when you were 18, no?

I read a book written in the US in 1961 about reliability of
electronics in the US military. On an aircraft carrier there were
12,500 tubes and 250 guys to look after them, and they were not
loafing about. Always something breaking down and needing a fix. The
books said had we had the 3rd world war we could have had, there would
have been only a 1/3 of a war because 2/3 of the really fancy rockets
and stuff would have done nothing after pressing the button, exploded
right there and then, or exploded just after launch. 1/3 of the Navy
was immobile at any given time on average due to "technical problems"
mainly due to poor electronics which were mostly over designed, ie,
expected to do more than it could for far too long.

I guess there was an upside. The Russians probably had more bothers
than the US. And with only 1/3 of a war, ppl would have been only 1/3
killed.

All the big powerful nation states soon learnt to welcome a world run
on Solid State.

Patrick Turner.



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Default Gilbert Cell AM modulator and demodulator.

On Nov 17, 10:38*am, Big Bad Bob BigBadBob-at-mrp3-
wrote:
On 11/15/10 19:26, Patrick Turner so witilly quipped:

There is a simple mix of RF voltage and AF voltage applied to an SE
pentode ( 6BX6/6EJ7), biased for normal SE class A operation.


The RF signal makes the pentode amplifiy it across a tuned tank in the
anode circuit.
The AF signal alters the grid biasing of the pentode thus changing the
gm of the tube between being twice the idle value and being almost nil
at cut off. The voltage gain of a pentode = gm x RL. The RF signal
current is modulated by the change in tube gm that is caused by the AF
change.


if you're not ACTUALLY using a pentagrid tube as oscillator + modulator
you might get away with re-mixing phase-inverted carrier back with the
modulated signal, giving you sum + diff + MUCH LOWER CARRIER. *Using a
spectrum analyzer you could then determine when you have 100%
modulation, and the THD would be MUCH LOWER since you would be able to
restrict operation to a more linear part of the modulation tube's gain
curves.


Nice idea but I don't have a spectrum analyser. I just got eyes, and a
good THD measurer.

I just wanted to make sure I could generate an AM signal simply which
went as close as possible to 100% mod and have THD under 1%, and
rapidly reducing once moving away from 100%.

The modulator with one pentode turned out to be better than I thought.
The receiver I was working on with 6J8 mixer and 6U7 IF amp had a
tendency to create what seemed to be a slightly over modulated IF wave
when my sig gene signal definately was just under 100%. Plus on weak
signals the detector diode threshold and flattening effects near 100%
mod create a lot of distortion.
The set really needs an RF input amp stage for better SW reception to
get a higher IF signal. Can't have everything.

You would need to have some pretty well aligned gear to make
this work, but with an AGC circuit built into the carrier oscillator (so
the output is very consistent) and careful design of the modulator, you
could get away with it. *You'd effectively have a hi-fi AM modulator,
made with 3 or 4 RF tubes and a handful of additional components.


Hmm, I'd also need a month's worth of sundays and then some methinks.

For strong local AM stations the detector I do use seems to do all I
ask when modulation is below 90%.


Patrick Turner

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Default Gilbert Cell AM modulator and demodulator.

On Nov 17, 10:51*am, flipper wrote:
On Tue, 16 Nov 2010 15:38:50 -0800, Big Bad Bob





wrote:
On 11/15/10 19:26, Patrick Turner so witilly quipped:
There is a simple mix of RF voltage and AF voltage applied to an SE
pentode ( 6BX6/6EJ7), biased for normal SE class A operation.


The RF signal makes the pentode amplifiy it across a tuned tank in the
anode circuit.
The AF signal alters the grid biasing of the pentode thus changing the
gm of the tube between being twice the idle value and being almost nil
at cut off. The voltage gain of a pentode = gm x RL. The RF signal
current is modulated by the change in tube gm that is caused by the AF
change.


if you're not ACTUALLY using a pentagrid tube as oscillator + modulator
you might get away with re-mixing phase-inverted carrier back with the
modulated signal, giving you sum + diff + MUCH LOWER CARRIER. *Using a
spectrum analyzer you could then determine when you have 100%
modulation, and the THD would be MUCH LOWER since you would be able to
restrict operation to a more linear part of the modulation tube's gain
curves. *You would need to have some pretty well aligned gear to make
this work, but with an AGC circuit built into the carrier oscillator (so
the output is very consistent) and careful design of the modulator, you
could get away with it. *You'd effectively have a hi-fi AM modulator,
made with 3 or 4 RF tubes and a handful of additional components.


So, if I understand, you're suggesting to limit initial modulation
depth to a low distortion range, say 75%, and then subtract 25%
carrier so the result appears 100%.

Did I get the idea right?- Hide quoted text -


Easier said than done.

Patrick Turner.

- Show quoted text -


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Default Gilbert Cell AM modulator and demodulator.

flipper wrote:
Which makes me wonder. In the method you suggested wouldn't there be
'exaggerated' side bands?


hmmm... not really. The idea would be to add enough carrier back in to
balance the sidebands. It proceeds on the assumption that the output of a
balanced modulator will ONLY be the sidebands, and then you add carrier back
into produce the AM signal, with the levels assigned such that you operate
ONLY in the linear range of the balanced modulator. I guess this is a
similar concept to class 'A' output stages in push-pull amplifiers. By
staying within the linear range, you minimize THD, and still get the overall
benefit of a push-pull output, but output power is reduced and dissipated
power is much higher.

So in short an AM signal consists of the center (carrier) frequency, and the
sidebands (carrier +/- modulation frequencies, in proportion to the input
signal). As I recall 100% modulation gives you 50% carrier and 25%
sidebands. This is why SSB transmission is so effective, since you get
equal range for only 25% of the power. But for broadcast AM you need the
carrier, so you simply "add it back in" and you're done.

If you think of a modulated carrier in terms of a Fourier transform it
starts to make more sense. The modulated signal is simply a summation of
sin and cos functions, each with different frequencies.

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Default Gilbert Cell AM modulator and demodulator.

On 11/17/10 04:44, Patrick Turner so witilly quipped:
But for FM audio detection, there is a 19kc pilot which is always
present and of sufficient amplitude that it can be fed into a second
input of an oscillator to phase lock the oscillator so that the 38kHz
result after a doubler does the business of converting DSB to being
easily detected with diode& CR. But far more thought and application
has preceeded me and I don't have time for everything, or to re-invent
the wheel.


I had an old tube FM stereo receiver at one time that did JUST THAT
(doubled the 19khz for 38khz), and after doing simple AM detection for
the resulting L-R signal, used a pair of 12AT7s as a single stage
balanced demodulator to generate the L and R from the L+R and L-R
signals. As I recall the L-R signal is LSB 38khz and along with the
19khz pilot provides you with a single side band + carrier (now easily
demodulated). All of that has worked pretty well over the years and
limits total signal bandwidth to 38khz, which works well for +/- 100khz
bandwidth for FM modulation. That works out to 1st and 2nd harmonics
for the stereo signal (and maybe a bit of the 3rd), and at least 5th
harmonic for everything else.

Does anyone know how to calculate theoretical THD from the harmonic
bandwidth of an FM modulated signal? I sure don't...

I read a book written in the US in 1961 about reliability of
electronics in the US military. On an aircraft carrier there were
12,500 tubes and 250 guys to look after them, and they were not
loafing about. Always something breaking down and needing a fix. The
books said had we had the 3rd world war we could have had, there would
have been only a 1/3 of a war because 2/3 of the really fancy rockets
and stuff would have done nothing after pressing the button, exploded
right there and then, or exploded just after launch. 1/3 of the Navy
was immobile at any given time on average due to "technical problems"
mainly due to poor electronics which were mostly over designed, ie,
expected to do more than it could for far too long.


the Vietnam war era was a LOT like that. By the time my era of
experience rolled around (1980s) reliability was significantly better
(we were testing tomahawk cruise misiles, and they performed extremely
well).

Then there was a story I heard from a friend of mine who was an aircraft
ET during Vietnam. Apparently the F4s had a particular tyupe of radar
that had to be re-calibrated before each mission, and the calibration
would drift within a couple of hours to the point where it was no longer
usable, and that REALLY sucked if the mission went longer than an hour
or two. My friend was able to align the equipment so that it would
drift across the specs from one end to the other and stay working for
WAY longer than 2 hours. And so the pilots always asked for HIM to
align it. Anyway, that kind of equipment familiarity and common sense
tweeking was definitely encouraged (somewhat surprising to someone who
hasn't "been there", I bet). You might say that a good portion of
otherwise faulty equipment was kept alive because of intelligent,
well-trained techs.

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Patrick Turner Patrick Turner is offline
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Default Gilbert Cell AM modulator and demodulator.

On Nov 27, 7:00*pm, Big Bad Bob BigBadBob-at-mrp3-
wrote:
On 11/17/10 04:44, Patrick Turner so witilly quipped:

But for FM audio detection, there is a 19kc pilot which is always
present and of sufficient amplitude that it can be fed into a second
input of an oscillator to phase lock the oscillator so that the 38kHz
result after a doubler does the business of converting DSB to being
easily detected with diode& *CR. But far more thought and application
has preceeded me and I don't have time for everything, or to re-invent
the wheel.


I had an old tube FM stereo receiver at one time that did JUST THAT
(doubled the 19khz for 38khz), and after doing simple AM detection for
the resulting L-R signal, used a pair of 12AT7s as a single stage
balanced demodulator to generate the L and R from the L+R and L-R
signals. *As I recall the L-R signal is LSB 38khz and along with the
19khz pilot provides you with a single side band + carrier (now easily
demodulated). *All of that has worked pretty well over the years and
limits total signal bandwidth to 38khz, which works well for +/- 100khz
bandwidth for FM modulation. *That works out to 1st and 2nd harmonics
for the stereo signal (and maybe a bit of the 3rd), and at least 5th
harmonic for everything else.

Does anyone know how to calculate theoretical THD from the harmonic
bandwidth of an FM modulated signal? *I sure don't...


Just try it and measure it. Then wysiwyg, no?

But the idea of the stereo demodulator you speak might suffer problems
of getting less stereo separation as F rises above about 2kHz due to
pre-filtering and adding-subtracting audio F only.

I tried something like what you say and could never get good channel
separation past about 20dB.

But maybe you should see this schematic of mine where there is a diode
matrix to get L and R signals.

http://www.turneraudio.com.au/am-fm-...ex-decoder.htm

See about 3/8 down the page.

The 19kHz pilot tone is prevented from entering the diode matrix where
it will form IMD products.
The pilot is however well filtered with two tuned circuits and applied
to an amp and transformer which has two diodes to form a 38kHz carrier
which is then applied to one grid of a two input locked or
synchronised oscillator. The purified 38kHz is then applied from the
tuned 38kHz transformer to the diode matrix and out come L and R
signals without much pre-filtering.

The circuit is based on the Quad FM MPX decoder which was originally
done using 3 germanium transistors for circuit in a box able to be
screwed to the rear of a Quad FM mono tuner. But I used the tubes as
shown instead, but just applied the Quad principle. I had to hand wind
several coils and transformers.

The down side of this method is that there are many tuned circuits
which have to be tuned **just right** to get phase coherence of 38kHz
sub carrier and applied carrier generated in the set. The tuned
circuits drifted with temperature to make the stereo separation wander
considerably. But when adjusted right it gave very good stereo.
But the whole set was not as quiet as *any" silicon MPX chips could
do, and the ICs could produce wider AF bandwidth than the tubes and
with less IMD at the top end.

I could never figure why there was more noise in stereo than in mono.


I read a book written in the US in 1961 about reliability of
electronics in the US military. On an aircraft carrier there were
12,500 tubes and 250 guys to look after them, and they were not
loafing about. Always something breaking down and needing a fix. The
books said had we had the 3rd world war we could have had, there would
have been only a 1/3 of a war because 2/3 of the really fancy rockets
and stuff would have done nothing after pressing the button, exploded
right there and then, or exploded just after launch. 1/3 of the Navy
was immobile at any given time on average due to "technical problems"
mainly due to poor electronics which were mostly over designed, ie,
expected to do more than it could for far too long.


the Vietnam war era was a LOT like that. *By the time my era of
experience rolled around (1980s) reliability was significantly better
(we were testing tomahawk cruise misiles, and they performed extremely
well).

Then there was a story I heard from a friend of mine who was an aircraft
ET during Vietnam. *Apparently the F4s had a particular tyupe of radar
that had to be re-calibrated before each mission, and the calibration
would drift within a couple of hours to the point where it was no longer
usable, and that REALLY sucked if the mission went longer than an hour
or two. *My friend was able to align the equipment so that it would
drift across the specs from one end to the other and stay working for
WAY longer than 2 hours. *And so the pilots always asked for HIM to
align it. *Anyway, that kind of equipment familiarity and common sense
tweeking was definitely encouraged (somewhat surprising to someone who
hasn't "been there", I bet). *You might say that a good portion of
otherwise faulty equipment was kept alive because of intelligent,
well-trained techs.


Indeed. And no, I wasn't called up for military service, so I didn't
go to Nam, and I would not have even if forced.
My parents gave me full backing as thay were fully against Australian
involvements in a civil war.
I/2 of all Ox was firmly against the war, even though many had fought
hard against Hitler and ****ing Tojo.

But I had friends who happily went to Nam, and they often didn't like
talking about it, but I never held it against them. I had to work with
them in civi land. I just believed that anyone was entitled to do as
they wished on the matter but pardon me if I don't go. I saw the film
Dogs Of War made years later by Robert McNamara who later vindicated
my desire to be absent from the Nam. Anyway, its all over now, and the
Commies won even though we killed many more of them than they killed
of us.

But yes, you're dead right about service ppl developing skills and
tweaks that are not listed in the service manuals. In Nam they must
have saved many lives.

After the war ended in 1975, the commies wanted revenge on southern
sympathisers. Oz took in over 200,000 Viet refugees, and gladly. They
have been a wonderful boon to australia with many of their kids
becoming highly skilled and very useful happy ppl.
They put the damn war behind them.

The commies have quietly junked many of the fuctard socialist ideas
they held so dear in their minds and against which we fought so
bravely but in vain to defeat lest western business interests in SE
asia be threatened by the spreading disease of socialism.

But despite sanctions against Vietnam for many years after 1975, the
Viets have gone rather capitalistic, with socilalist rule - for now.
In another generation the old Ho Chi Min brigade will have died right
out and nobody will bother about granpa's bravery at a silly war where
such an enormous waste went on. They'll all just get on with business
and work.

A large number of now ageing Vietnam veteran soldiers here have never
recovered from the trauma of being there. I met quite a few who said
the US should have A-bombed Hanoi, while other hated the lack of
recognition of their heroic contributions to their beloved country.
Some hated the crap behaviour they saw each day in Vietnam, like the
effects of Agent Orange. The anger in Oz over Vietnam continues to
this day for many different reasons and is for many an unresolvable
dilemna, and they ask "please, pass me that bottle, it helps me
forget."

Now the allies of the North Viets, the Chinese, are our favourite
trading partner and we are excavating huge holes on Oz and sending
them mountain-fulls of minerals so they can have an apple pie & cream
life wev'e had for so many more years than them.
99% of ppl here wear ****ing Chinese underpants, socks, shoes, suits,
and our younger generation has no memory of an Australia which was
largly self sufficient for all of its goods and services which were
provided by folks here who really knew the meaning of hard work. The
irresistable ingredient is the dirt cheap prices of the China imports.
Someone knows the meaning of hard work for a pittance and it ain't us.
Now though, China don't need to invade us, they'll just buy a mountain-
full or hole-full at a time, damn cheap and efficient, especially if
they buy the mining company. So globalisation seems to have kept
nations away from wars like the one Japan started at Pearl harbour.
How come nobody thought of doing things differently in 1933? We are
indeed a BS species, with a big mix of inconsistent behaviours.
But we have a huge education industry here where our universities
teach many thousands of international students. These will go back
home and remember their time here and our democracy, and hopefully the
AK47 their granpa has stored away "just in case" will rust to bits
before it is ever used again.

Ingenuity with tricky old electronics isn't needed now. Just chuck it
out an buy another. Replace the module. Don't analyse anything. Nodody
will pay you unless you fix the problem by yesterday. Anything else is
"un-professional" Patience is a vice, not a virtue. Slowly things like
books and teachers are becoming as useful as a stuffed turkey. Just
get rid of both, and Google it, and you'll know. Modern BS.

Nevertheless, some areas of activity do need a mountain-full of
practical ingenuity, say in medicine for example. Ya don't want some
dude Googling to find out what to do about your broken leg.

Patrick Turner.









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