On 8/2/2014 10:52 PM, Lostgallifreyan wrote:
Jerry Stuckle wrote in
:

That's useful. It ties in with things I have read about phase
modulation synthesis. At risk of opening a wild (but fun) bit of
off-topic, I wonder if given a reference carrier at precise fixed
frequency, a phase modulation technique might allow advantages to
signal transmission that FM lacks.


Not really. All phase modulation does is integrate the modulating
signal and use that to shift the phase of the carrier (which also
requires a frequency change to effect the phase change). The resulting
signal is exactly the same as if the carrier were directly frequency
modulated. The only difference is how you got there.


Ok. What made me wonder is that I read (in the context of phase modulation
for musical audio synthesis) that true FM methods can cause a frequency drift
that is avoided completely by using phase modulation. I wondered if that fact
might (at some other cost) be useful in signal transmission at RF.


Yes and no... Without knowing the context, I would guess the frequency
drift they refer to is caused by a DC bias to the modulator from the
incoming signal itself with FM modulation. This would assume the
incoming signal is DC coupled, which I would guess it might be in music
synthesizers (don't know - never worked with one).

Using phase modulation, if there were a DC bias on the signal, it would
just old the phase at a constant angle - and since frequency shift is
based on the change in the phase angle of the signal (and vice versa, of
course), the signal would have zero deviation.

It's something I've never encountered in radios, but then you don't
normally see radios DC coupling the audio signal to the modulator. But
I will admit most of the FM transmitters I've worked with (both
commercial FM and VHF/UHF business and ham bands) have been phase
modulated.

The only time I remember getting deeply into direct FM was back in the
early to mid 70's when I was working on a frequency synthesizer of 2
meters (this was back when crystal rigs were still common). It's pretty
easy to get FM out of such a circuit since frequency is directly
controlled by voltage - all you need to do is feed the modulating signal
into the VCO feedback loop. It ended up working pretty well, and I
didn't have any problem with frequency drift (but then with a VCO any
drift automatically solves itself).

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