--- Jeff DePolo WN3A <[EMAIL PROTECTED]> wrote:
>
> > 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
>
I think we're on two different wavelengths here. What
I am talking about is practical application, not
theoretical mumbo-jumbo. Going back to the first post,
Bob was talking about generating a .01 Hz PM signal
with a function generator- thats all well and good,
but we don't use function generators for transmitters.
And what a function generator can do with a single
tone is different that what a "real" modulator can do
over 8 decades of frequency change.
I'm not arguing the point about "theoretical" PM. It
is as you say, good down to approaching DC, but not
DC. What I am trying to say is that I haven't seen a
PM modulator with that kind of range,(.01 to 3000Hz)
but I see FM modulators that can do that and fit in
your hand and cost $100. There may be some broadcast
PM TX out there, I haven't seen it, and I don't know
what it might cost. (probably more than $100).
Reference your points:
> 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).
What I am talking about is a single input modulator
(just like FM), where you put your audio in at a
certain point, and it works. There are too many
decades of audio for a single simple PM modulator (ala
FM) at a 20 dB per decade increasing rate, which
requires 20 dB less of audio per decade. There is over
100 dB of change from .01Hz to 3KHz. If you start at
the top of the dynamic range of the amplifier feeding
the modulator, at .01Hz, you'd be down a 100 dB by the
time you get to 3Khz. And for good sounding NBFM you
need at least 50 dB SNR at that point. That means the
amp and the modulator have to have a dynamic range of
at least 150 dB. Can this be done? Sure, but in a
"real" world environment it becomes difficult to
impossible, at a price people will pay.
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.
Maybe so, I haven't seen them. You probably have
access to more of that stuff than I do.
> 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.
I guess I'm the only one talking real world. Who cares
what theoretical says if it takes a microprocessor and
banks of filters/DSP to make it work?
> 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...
>
But compared to a volt P to P that is a small signal.
We wouldn't run volts of audio down the same system,
with no changes. You wouldn't put speaker level audio
on a line designed for 10 uV, and expect everything to
play fine. 100 dB of dynamic range is about the best
we can expect out of a good CD changer. Another
consideration is the impedence of the signal path. 10
uV is not bad on a 30-200 ohm circuit, but most
modulators are up in the Kohm range.
Anyway, what I am talking about is real world
limitations on the theoretical PM. Sure theoretically
you could build a modulator that would do .01 to 3
KHz. Would it be expensive? duh. Would it be complex?
duh. Would it work any better than a simple FM
exciter? Probably not. That's my point, sorry if
people think I'm knocking "theoretical" anything. I
have learned to be cautious when you say "never" and
"always".
Joe
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