Phase modulated carrier thru rf amp tank circuit??
 

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

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

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

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

"Inertia" is a relative thing. Consider a one-cylinder motor with no
flywheel at all (imagining it could still run). The crankshaft rotation
would be very jerky, wouldn't it? Now put a small flywheel on it. The
jerks wouldn't be as abrupt, but the rotation would still be jerky. As
you make the flywheel bigger and bigger, the jerks smooth out, but the
flywheel has to get really big before the rotational speed becomes, for
all practical purposes, constant, without varying some during each
rotation. That's a pretty good analogy. A low-Q tank circuit is like the
little flywheel, and a high-Q tank like a big flywheel.

A tank that provides the other half of an RF cycle does interfere with
abrupt phase changes. But the circuit can usually be designed to provide
enough restoration of the carrier sine wave while retaining enough of
the modulation characteristic to be useful. Also, a single-resonator
tank circuit isn't the only trick in the engineer's bag. More complex
filters, such as multiple pole bandpass and lowpass filters, can be
designed that are much more selective in what they do than a simple
single LC tank circuit.

The larger and more abrupt the changes, the more careful and clever the
designer has to be. But the design of wideband modulation systems is
well within the capabilities of a competent RF engineer.

If you have an oscilloscope and a signal generator capable of being
frequency modulated, you can run some experiments with LC circuits and
filters that should be quite educational. And a spectrum analyzer would
enhance the educational value considerably.

Roy Lewallen, W7EL

gary wrote:
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

"Inertia" is a relative thing. Consider a one-cylinder motor with no
flywheel at all (imagining it could still run). The crankshaft rotation
would be very jerky, wouldn't it? Now put a small flywheel on it. The
jerks wouldn't be as abrupt, but the rotation would still be jerky. As
you make the flywheel bigger and bigger, the jerks smooth out, but the
flywheel has to get really big before the rotational speed becomes, for
all practical purposes, constant, without varying some during each
rotation. That's a pretty good analogy. A low-Q tank circuit is like the
little flywheel, and a high-Q tank like a big flywheel.

A tank that provides the other half of an RF cycle does interfere with
abrupt phase changes. But the circuit can usually be designed to provide
enough restoration of the carrier sine wave while retaining enough of
the modulation characteristic to be useful. Also, a single-resonator
tank circuit isn't the only trick in the engineer's bag. More complex
filters, such as multiple pole bandpass and lowpass filters, can be
designed that are much more selective in what they do than a simple
single LC tank circuit.

The larger and more abrupt the changes, the more careful and clever the
designer has to be. But the design of wideband modulation systems is
well within the capabilities of a competent RF engineer.

If you have an oscilloscope and a signal generator capable of being
frequency modulated, you can run some experiments with LC circuits and
filters that should be quite educational. And a spectrum analyzer would
enhance the educational value considerably.

Roy Lewallen, W7EL

gary wrote:
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

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

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 Roger and Roy. I think I'm starting to get it. As the data
rate gets higher the tank becomes more of an issue. The PSK31
discussion really opened my eyes.

73
Gary
W4AF


"Roger Leone" wrote in message ...
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 Roger and Roy. I think I'm starting to get it. As the data
rate gets higher the tank becomes more of an issue. The PSK31
discussion really opened my eyes.

73
Gary
W4AF


"Roger Leone" wrote in message ...
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

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.

A sudden phase shift like that (within one RF cycle) only happens if you phase
modulate the carrier with a square wave, but then the tank circuit will even
things out, but will still be quite a wide bandwidth.

You don't modulate any carrier in any mode using square waves to be honest -
unless you pass that carrier thru a good filter after modulating it and before
letting it loose. You should low-pass filter the modulating signal before
letting it modulate the carrier - that way the phase change will be slow and not
sudden as you were thinking.

Clive

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.

A sudden phase shift like that (within one RF cycle) only happens if you phase
modulate the carrier with a square wave, but then the tank circuit will even
things out, but will still be quite a wide bandwidth.

You don't modulate any carrier in any mode using square waves to be honest -
unless you pass that carrier thru a good filter after modulating it and before
letting it loose. You should low-pass filter the modulating signal before
letting it modulate the carrier - that way the phase change will be slow and not
sudden as you were thinking.

Clive

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)