In article ,
flipper wrote:
On Sun, 04 Sep 2011 20:25:25 -0500, flipper wrote:
snip
Some sort of modulation meter would be more useful but neither easy
nor particularly inexpensive, compared to the rest of it.
Well, I kludged up a simple 'modulation indicator' in spice, but I
haven't actually built it.
It's posted on the web page so see what you think.
http://flipperhome.dyndns.org//Twin%...ransmitter.htm
The commonly obvious thing would be to make a 'LED VU' meter with
something like the LM3915 but the modulator drive has an interesting
problem in that trying to sense 'max' positive mod may not be so
simple due to positive grid drive potentially clipping the signal. It
shouldn't if everything is 'right', but it might not be, and then
there's the matter of 'calibrating' the thing even if all else is
okay.
Positive mod isn't the 'real' (or biggest) problem anyway, it's
carrier cutoff on negative mod, and 0 modulator current has the
advantage of being 0 no matter what 'idle' or peak positive mod is. So
I look for modulator cathode approaching 0, rather than worry about
'calibrating' to positive mod, then peak hold/decay it for visual.
(Note, the LM3915 doesn't peak hold so you could easily miss seeing
carrier cutoff events even if you got it calibrated right)
The yellow 'mod' LEDs are for visual effect. The R15/C19 filter senses
'idle current' and U1B, then, generates (rectified half sine) pulses
proportional to positive mod to drive the LEDs. We don't care if it's
terribly accurate as it's sort of like those 'musical lights' things
where light intensity dances around to the music.
The idea is a sort of 'glow indicator'. Like, maybe, aiming the LEDs
into a diffuser, or not. I haven't worked out the details. Probably
need to buy some 'wide angle' flat LEDs and see just how well they
disperse and how it looks.
In theory it would be nice to know our peaks are -3dB, or whatever,
but for something this simple I think people would slam it as high as
they can anyway, so a red 'too much' indicator is probably sufficient,
and calibration, as mentioned, would be non trivial so it's unlikely
-3dB would have much meaning to the actual problem. At least that was
my thinking: a modulation 'idiot light', to use a car analogy.
A simple tripler off the heater supply should be enough to power it.
It's an interesting coincidence that you are working on this idea, as I have
been thinking along similar lines the past couple of days.
As you allude to above, I think the important thing would be to monitor the
onset of oscillator dropout and flash warning indicator at that point.
The idea that struck me was to use a neon bulb like some 1950s era tape
recorders used as recording level indicators. To a first approximation a neon
bulb and two resistors would do the job. One connection of the neon bulb would
be go to the modulator plate, with the second connection to the junction of two
resistors forming a voltage divider between B+ and ground. The potential at the
junction of the two resistors would set the firing voltage for the neon bulb,
and the fact that the firing voltage is higher than the conduction voltage would
provide a sort of faux peak hold effect. Calibration of this circuit should be
easy as we only have to identify the point where the oscillator drops out, not
the precise point of 100% modulation.
There is at least one obvious problem with this elegantly simple scheme, which
is that when the neon bulb fires it would drag the modulator plate down to a
lower voltage, creating serious distortion. This problem could be overcome at
the cost of an additional triode operating as a cathode follower to drive the
neon bulb, buffering the modulator plate. Unfortunately besides requiring the
extra triode, an additional higher voltage power supply would be required for
the cathode follower plate, to prevent it from saturating before the point where
the oscillator drops out.
Most old vacuum tube era professional AM broadcast modulation monitors required
calibration vs. an external source such as Patrick's trapezoidal CRO display.
There was one old modulation monitor though which was self calibrating, that was
the Gates M5693 Modulation Monitor. The ideas from this monitor should be easy
to adapt to the modern solid state opamp era. A year ago I worked out a scheme
to implement this self calibrating idea in a single quad opamp to drive an
analog peak modulation meter. A little additional circuitry, as in the actual
MS5693, could provide for driving a red peak flasher LED, and the analog meter
could be replaced with an orange LED "VU" meter.
An AM demodulator would be required to drive this circuit, the oscillator plate,
which is a lower impedance point than the grid circuit where the antenna is
connected, might provide enough RF drive voltage for a demodulator diode without
unduly loading the oscillator circuit.
If you want to puzzle out the self-calibrating principle used in the M5693, the
operating instruction manual and schematic can found here.
http://louise.hallikainen.org/~harol...GatesM5693.pdf
--
Regards,
John Byrns
Surf my web pages at,
http://fmamradios.com/