--- [EMAIL PROTECTED] wrote: > Hi Joe, > > Fact: PM does not reproduce DC well. Actually, it > doesn't reproduce DC > at all (beyond a spike) because in DC there is no > change in the phase of > the signal. The closer you get to DC, the worse PM > will perform. FM, on > the other hand, has no such limitation. > > As a blanket statement, the above is not true. I can > generate a beautiful 10 > Hz or 1 Hz or 0.01 Hz phase modulated output at any > deviation you want using > one of the newer function generators. It's done with > digital techniques and not > with a varicap and LC tank because the deviation > simply isn't obtainable with > the latter. But nevertheless, the theory contains no > such low frequency > limitation and there is equipment on the market that > performs the function just > fine. ---snip---- > 73, > Bob >
I'd like to look at the above statement for just a minute. Since we're all in agreement that PM has a 6db per octave inherent "pre-emp" (that would be 20 dB per decade), let's look at the numbers in something other than a "theoretical" light- ie; real numbers. 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. It would therefore take 10 volts P to P to modulate the 10Hz tone, and 100 volts P to P to modulate the 1 Hz tone. If we went down to .01 Hz (as mentioned above), it would take 10,000 volts P to P to modulate the exciter! Personally, I don't want to be working with such voltages, thats higher than a 2KW linear has in it. So, even theoretical PM falls apart when you get down to really low modulating frequencies. Most audio amplifiers can't handle that much dynamic range, they are good over just 2 to 3 decades or so with a 6 dB per octave slope, while maintaining good signal to noise ratio. The problem is not with generating just a specific tone freq, but the range of frequencies needed. Fortunately, the voice band only covers one decade, and the PM exciter can handle that OK. You might be able to get a second decade (down to 30 hz) to work OK. That third, fourth, fifth, and sixth decade that the FM exciter could easily reproduce get very difficult on a PM transmitter, due to the additional 20 dB of audio needed every time you drop a decade. 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/
