From: spam@spam.com   
      
   On Sun, 17 Jul 2011 17:06:09 -0500, John Byrns    
   wrote:   
      
   >In article <4e22c684.16025155@news.eternal-september.org>,   
   > spam@spam.com (Don Pearce) wrote:   
   >   
   >> On Sun, 17 Jul 2011 04:14:48 -0700 (PDT), Patrick Turner   
   >>  wrote:   
   >>   
   >> >I tried full wave, but not so easy at all. I've tried a 2 diode   
   >> >voltage doubler, also not worth the slight increase in Vo.   
   >> >   
   >> >There is utterly no need for any more than what I have, IMHO.   
   >>   
   >> Have you tried a synchronous detector? Generate a well-limited square   
   >> wave at the IF frequency and use it to switch a full wave diode   
   >> bridge. This will recover - with perfect linearity - any AM signal,   
   >> even one modulated well beyond 100% (provided it has been carried out   
   >> properly).   
   >   
   >Neglecting the problems with the limiter around the zero carrier point, your   
   >scheme doesn't work for signals "modulated well beyond 100%" in the negative   
   >direction.  The problem is that when modulation exceeds 100% in the negative   
   >direction the phase of the carrier flips causing the switching of the diode   
   >bridge to be out of phase with the original carrier, causing serious   
   distortion   
   >in the receiver.   
   >   
      
   That is precisely the circumstance in which the synchronous detector   
   scores. Over-100% modulation is recovered correctly. With a simply   
   diode detector, of course, it simply clips.   
      
   >Adding a phase detector and a VCO to your circuit will fix this problem.   
   >   
      
   Phase detector and VCO are a good idea, but not needed unless the   
   carrier genuinely vanishes occasionally. I would hope most   
   broadcasters try to avoid this. Actually, if the carrier has vanished   
   there is nothing to be gained with a VCO - there is nothing to detect   
   in the gap.   
      
   >Extra credit quiz question, what tube era broadcast transmitter could generate   
   >negative modulation peaks beyond 100% if the negative peak clipper in the   
   >transmitter was disabled?   
      
   Any that used a double balanced modulator and linear output stage   
   rather than an anode current modulator.   
      
   d   
      
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