--- Jeff DePolo <[EMAIL PROTECTED]> wrote: > > Real modulator voltages have to start somewhere as > a > > reference. If we take .1 volt P to P to deviate a > PM > > exciter to 5KHz of deviation at a 1KHz audio tone, > > then it will take 1 volt P to P to drive the same > > modulator to 5KHz deviation at 100 Hz of audio. I > > think everyone will agree with this so far. > > You're limiting "real" modulators to varactor > designs? Just because > that's what *some* of our aging land mobile radios > use doesn't make it > the only kind of PM modulator. > > > So, even theoretical PM falls apart when you get > down > > to really low modulating frequencies. > > No, you're wrong, theoretical PM doesn't fall apart > until you reach DC. > Real-world implementations are what might fall > apart. Math doesn't fall > apart at low frequencies. > > For your varactor example I agree with your math > Joe, and that it would > be impractical to get 0.01 Hz response out a > run-of-the-mill two-way > mobile, but that's about as far as you can go > without crossing the line > between practical and theoretical. > > --- Jeff
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. Joe __________________________________ Do you Yahoo!? Yahoo! Tax Center - File online by April 15th http://taxes.yahoo.com/filing.html Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
