> I guess I'm not following your logic. If you could get
> a PM exciter to produce a .01 Hz tone at 5 KHz
> deviation, the amount of audio required at 1 Hz would
> be 40 dB below that. The amount of audio required to
> modulate 5Khz deviation at 1KHz tone would be 60 dB
> below the 1 Hz level, or 100 dB below the .01 Hz
> level.
>
> This means that a modulator that could produce a .01
> Hz tone at 5 KHz of modulation for 1 volt P to P would
> only require 10 micro volts P to P at a 1 KHz tone.
> Thats a very small level in anyone's book, and the SNR
> would be garbage, since most audio type amps only have
> 120 to 130 dB of maximum dynamic range.
>
> I'm not limiting the PM designs to varactor. Any
> "true" PM modulator has a 6 dB/octave curve, and
> therefore falls under this calculation. If the audio
> deviation doesn't increase by 6 dB/octave, you don't
> have a "true" PM.
1. It seems that you keep reverting to logic that is bound by limitations
in audio dynamic range and S/N rather than sticking to what we were
originally discussing - the theoretical capabilities of PM (which, again,
has an LF cutoff of DC).
2. You can create PM via digital techniques that wouldn't be constricted by
analog world limitations like dynamic range and S/N. This isn't "black
magic"; modern FM broadcast exciters have been doing this for years.
3. Even in an analog design, there's nothing to say that you have to have
one audio path/stage that the audio passes through. You can have a
low-level amplifier that is used for high frequencies (those requiring less
amplitude), and a high-level amplifier for lower frequencies, the output of
which two would be summed prior to the modulator. Again, this is a moot
point since we're debating the theoretical capabilities of PM, not the
real-world implementations.
4. Regarding your comment that 10 microvolts P-P is a very small level and
the SNR would be garbage, a typical dynamic mic has an output around -100
dBV (10 microvolts) at an SPL of 50. It's not a ridiculously-low audio
level to deal with in the AF domain...
One more time for the folks in the cheap seats - THERE IS NO THEORETICAL LF
LIMIT FOR PHASE MODULATION!
--- Jeff
--------------------------------------------
Jeff DePolo WN3A - [EMAIL PROTECTED]
Broadcast and Communications Consultant
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