RadioBanter - View Single Post - Phase modulated carrier thru rf amp tank circuit??

September 10th 03, 08:18 PM

(gary) wrote in message . com...
Can someone explain to me how a rf carrier that is phase shift
modulated, for lets say digital transmission, can be amplified in an
amp with a tank circuit output. Why doesnt the tank "flywheel effect"
prohibit or inhibit the abrupt phase changes in the signal. thanks

The ensuing discussion reminded me of when I learned about required
tank circuit bandwidths for some given modulation. Can you say, "hit
myself over the head with a hammer"? In this particular case, I
wanted a quick output pulse at 150MHz, and my first trial was to pulse
a lower-power stage in a multiplier chain. The output was anything
but a fast pulse! But it was still fairly wide bandwidth, compared
with what's needed in voice-bandwidth systems or probably even video.
The solution, after I realized what was going on (didn't take long...a
lot shorter time than it took to build the thing, for sure!), was to
pulse-modulate the screen of the power amplifier stage. Then the main
limitation to RF pulse rise time was the Q of the tank and the ability
of the power tube to drive that tank with high current pulses to
quickly build up the RF level.

So as others have noted, you WON'T change the phase 180 degrees in
half a cycle of the RF...it will take lots of cycles...but that's OK
because you don't NEED to change it that fast. For example, if your
digital bits last for, say, 1000 cycles of the RF, that would be a bit
time of 286 microseconds, or 3500 baud, at 3.5MHz carrier frequency.

Please note that at the receive end, you will do FAR MORE RF filtering
than on the transmitter end (in the RF tank circuits)! In fact, on
the transmitter end, you better band-limit the modulation signal
somehow, because the RF tank circuits will NOT limit it enough, in
general. So there will be filters in the modulation input path that
make SURE that you don't even try to change the phase that quickly
anyway. That's because such abrupt changes -- in phase or in
amplitude -- result in wide transmitted bandwitdh and cause
interference to other users of the spectrum. So the whole system,
including the modulation technique, data rate, transmitter, receiver,
and decoder/demodulator, work together to transmit the information in
"just enough" bandwidth to get the job done, and in general they DO
NOT use abrupt changes in phase or amplitude or frequency.

Cheers,
Tom