RadioBanter - View Single Post - Frequency multipliers: Usable modulation formats?

July 15th 05, 01:31 AM

From: (Michael Black) on Thurs 14 Jul 2005
16:24

"Joel Kolstad" ) writes:
Thanks for the detailed reply, Len... I'll sit down and work through some of
the math if/when I get a moment.

Could you tell which of the following is correct? -- Say I amplitude modulate
my voice onto a carrier and feed it through a frequency multiplier. Besides
shifting the carrier, does it just make me sound like a chipmunk (i.e., if I
manage to hum a perfect 1kHz sine wave, the demodulated signal is me humming a
2kHz sine wave with a 2x mulitplier)? Or is there far more serious distortion
present?

Virtually all multipliers we see work by overdriving a stage. This strips
off any amplitude modulation, so there's no modulation at the output of
the multiplier.

Well, that IS rather serious distortion...! :-)

True, the multipliers (or any following over-driven stage) will
strip off all the amplitude variations/modulations. But FM and
PM don't "modulate" the amplitude...even if the match APPEARS
that there are various amplitude sidebands present. Those
various resulting sidebands of FM/PM are the result of the
carrier "swinging" due to modulation and the various amplitude
sidebands are the RESULT of the constant swinging.

One of the ways of calibrating the DEVIATION of FM or PM, as
in checking a modulation monitor meter, is to (actually) look
for ZERO carrier frequency amplitude content, either by a
spectrum analyzer or a narrowband receiver. That's been an
accepted method of calibrating broadcast transmitter air
monitor meters for years and years. [for a hint on how that
is done, check them Bessel Function "zero-crossings" found in
many texts to derive the modulation test frequency and the
deviation]

Multiplying the carrier of FM/PM changes the DEVIATION of the
carrier (frequency for FM, phase for PM, a little of both
actually when there is some distortion happening). The rate of
change of the modulation itself has NOT changed, hence no
"chipmunk" voice sounds. Note that, all youse guys what is
locked into AM thinking. :-)

For a good treatment of the various modulation processes, I
suggest "Electronic Designers' Handbook" by Robert W. Landee,
Donovan C. Davis, Albert P. Albrecht, McGraw-Hill 1957, section
Five (45 pages). Vacuum tube oriented (note copyright of 1957)
but the modulation basics are there...the basics apply to all
active devices.

For "chipmunk sounds" (which I used to do for funsies at WRRR
many yarns ago) just use a tape deck with a different speed
capstan for play-back. The rate of change of modulation is
altered by the fractional (not integer) tape transport speed,
thus the change in voice frequencies. The same thing can be
done with recorded digital audio recovered with a DIFFERENT
clock rate on read-out compared to sampling/recording. That's
not just "multiplying frequencies" but rather changing the RATE
OF CHANGE of the WHOLE sound. Not the same as multiplying just
the carrier wave.

"Chipmunk" secret, the easy way: Use a 50 Hz capstan on the
Magnecord reel-to-reel to record, play back with the standard
60 Hz capstan to get the increased rate of change of sound.
Done in 1956, hardly any transistors on the market then,
certainly NO integrated circuits having on-board A-D things.
:-) Alas, reel-to-reel analog tape units are things of
antiquity in 2005. :-)