I still have trouble visualizing how a 180 or 270 degree change can occur
in a
single rf cycle and be able to overcome the "inertia" (probably a
poor choice of words) of the rf circuits , feed line and antenna
system.

Gary
W4AF


Gary:

For the phase shift to occur during one RF cycle, wouldn't that suggest that
the modulating frequency is close or equal to the RF carrier frequency? I
don't think that is the situation you are trying to visualize. The
modulating frequency, in voice or common digital modes, is more likely a
tiny fraction of the RF carrier frequency. The phase shift of the RF
carrier only has to occur at the modulating freqency, not at the RF
frequency. That means that during the modulating phase shift, many
thousands or even millions of RF cycles can occur.

If you are thinking of a mode like PSK 31, in which modulation is by phase
shift, remember that the phase shift occurs in the audio tone that is
modulating the RF signal. The RF signal can follow this phase change
easily, since many millions of RF cycles occur during the audio phase shift.

Roger K6XQ

Thanks for your response. I am struggling with this. I still have
trouble visualizing how a 180 or 270 degree change can occur in a
single rf cycle and be able to overcome the "inertia" (probably a
poor choice of words) of the rf circuits , feed line and antenna
system. These are large abrupt changes, not more suttle changes like
i would expect relative to voice modulation. I have read that in some
amplifiers the tank provides the other half of a single rf cycle. Why
wouldnt the same the action interfere with a phase shift in a rf
cycle. I could understand being able to detect a phase shift after a
given period of time with respect to a previous period. The period
being relatively long compared to the rf cycle time.

Gary
W4AF



Roy Lewallen wrote in message ...
Although the bandwidth of a phase modulated signal is theoretically
infinite, the vast majority of the energy is in a finite bandwidth. So
if you filter signal components beyond that bandwidth, you can still
recover the modulation information adequately. What you have to do,
then, is to design the tank circuit so its response isn't too narrow to
pass the modulation information. If it is too narrow, it will decrease
and distort the modulating phase shift.

If you were to build a tank circuit that acted as a perfect "flywheel",
i.e., had zero bandwidth, you wouldn't even be able to pass a code or
voice waveform through it -- everything would come out as a single
frequency, single amplitude sine wave, or nothing at all. (It would also
take an infinite time for it to respond to a signal.) For amplifying
some very broadband types of signals, tank and other tuned circuits are
avoided altogether.

Roy Lewallen, W7EL

gary wrote:
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
73

Gary
W4AF

Although the bandwidth of a phase modulated signal is theoretically
infinite, the vast majority of the energy is in a finite bandwidth. So
if you filter signal components beyond that bandwidth, you can still
recover the modulation information adequately. What you have to do,
then, is to design the tank circuit so its response isn't too narrow to
pass the modulation information. If it is too narrow, it will decrease
and distort the modulating phase shift.

If you were to build a tank circuit that acted as a perfect "flywheel",
i.e., had zero bandwidth, you wouldn't even be able to pass a code or
voice waveform through it -- everything would come out as a single
frequency, single amplitude sine wave, or nothing at all. (It would also
take an infinite time for it to respond to a signal.) For amplifying
some very broadband types of signals, tank and other tuned circuits are
avoided altogether.

Roy Lewallen, W7EL

gary wrote:
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
73

Gary
W4AF

Think of the phase shift as being a frequency shift (frequency can't vary
unless the phase shifts somewhere along the line. Phase modulation and
frequency modulation appear identical at the receive end.
(broke=not working, retired=not working, retired=broke)

Think of the phase shift as being a frequency shift (frequency can't vary
unless the phase shifts somewhere along the line. Phase modulation and
frequency modulation appear identical at the receive end.
(broke=not working, retired=not working, retired=broke)

(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

(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