On Mon, 09 Jan 2012 22:43:42 -0600, John Byrns
wrote:
In article ,
flipper wrote:
I've now added a pre-emphasis preamp so it's basically an operational
mono FM Transmitter.
http://flipperhome.dyndns.org/FM%20Stereo.htm
I was intrigued by the reactance tube technique and, by golly, it not
only works but I've now done one of my own.
Hi Flipper,
How stable and accurate is the center frequency of your reactance tube modulated
oscillator?
Really now, just how accurate do you think my fingers are when
squeeze-pull tuning that air coil? LOL
'Accuracy' isn't really an issue for me because it's intended use is
for old analog tuners anyway, although, my cell phone digital tuner
gets it okay.
I'm not really sure 'how good' the cell phone gets it because there's
distortion somewhere. I mean in any receiver, not just the digital. I
think it's in the modulator and am working on a cathode NFB scheme,
similar to what I did with the Twin Triode AM Transmitter, to see if I
can improve that.
Speaking of which, the 'Gort' AM Transmitters were a hit at Christmas.
Did you see the 'sales ad' I did for them? LOL
http://flipperhome.dyndns.org/Gort%20Docs.htm
An On/Off switch sounds so much sexier as a "Command Knob," don't you
think?
Is it good enough to work with a digital tuner that tunes in 200
kHz steps?
Dunno. My cell phone has 100KHz tuning.
This is all 'brave new world' stuff for me and, frankly, at this stage
I'm just tickled to death it oscillates where it should and that the
modulator actually modulates.
I have on old Jerrold CATV FM band modulator. I once drew a schematic, but who
knows what became of it. IIRC it used a 10.7 MHz oscillator modulated by a
reactance tube. The oscillator center frequency was stabilized by feedback from
a 10.7 MHz discriminator. The oscillator was then mixed with a crystal
oscillator to produce the final carrier frequency in the FM band. I wonder how
linear this scheme is, using a single reactance tube to modulate a relatively
low frequency oscillator a full +/- 75 kHz? Many old tube FM broadcast
transmitters used modulated oscillators around this same general frequency, but
developed the carrier frequency by using a chain of frequency multipliers which
increase the deviation along with the frequency, easing the demands on the
reactance tube modulator.
They also usually used double, push pull, modulators.
Jerrold modulating at 10.7 MHz gives me hope, though.
I would think a better scheme, than Jerrold's scheme, would be to run the
modulated oscillator at the carrier frequency, as you do, and then mix with a
crystal oscillator to get down to the 10.7 MHz discriminator frequency. Some
kind of buffer between the oscillator and antenna would also probably be
desirable to improve stability. Perhaps even running the modulated oscillator
at half the final carrier frequency, and using a doubler between the oscillator
and antenna as a buffer.
Yeah, I haven't gotten to the 'antenna' part yet. It spews quite a bit
as is.
The idea is simple, simple, simple, though, and I figured on little
more than a secondary off the coil. Well, and probably going to
something slug tunable rather than the boing boing air coil.
If it drifts it doesn't seem enough to bother analog tuners because
I've listened to it for hours without touching the dial.
I will have to dig out the old Jerrold, draw another schematic, recap it, and
try to get it running. One of your stereo coders might work nicely with the old
Jerrold.
I am flattered by the unfounded confidence
