From: spam@spam.com   
      
   On Tue, 19 Jul 2011 19:43:57 -0500, John Byrns    
   wrote:   
      
   >In article <4e25ba6f.2473286@news.eternal-september.org>,   
   > spam@spam.com (Don Pearce) wrote:   
   >   
   >> On Mon, 18 Jul 2011 16:20:55 -0500, John Byrns    
   >> wrote:   
   >>   
   >> >In article <4e2454b7.207434@news.eternal-september.org>,   
   >> > spam@spam.com (Don Pearce) wrote:   
   >> >   
   >> >> 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.   
   >> >   
   >> >No, you are completely wrong here!  First with negative modulation greater   
   >> >than   
   >> >100% a diode doesn't "simply clip", what it does, assuming a perfect diode   
   >> >detector, is recover the envelope of the modulated waveform.  What happens   
   >> >assuming a sine wave test signal, is that the portion of the signal that   
   >> >exceeds   
   >> >100% negative modulation flips upside down as seen by an envelope detector.   
   >>   
   >> Quite so, my bad.   
   >>   
   >> >Second what you are describing is what I believe is called a pseudo   
   >> >synchronous   
   >> >detector.  This type of detector responds in a similar manner as the diode   
   >> >detector does, although for slightly different reasons.   
   >>   
   >> No, what I am describing is an entirely synchronous detector - maybe I   
   >> described it poorly.   
   >   
   >I thought you described it perfectly well.  I suppose our difference stems   
   from   
   >how you define a "synchronous detector".  I define a "synchronous detector" as   
   >using a locally regenerated carrier that is essentially identical to the   
   carrier   
   >that was used in the transmitter.  The local carrier in your pseudo   
   synchronous   
   >detector doesn't meet my criterion because the phase of the locally   
   regenerated   
   >carrier flips every time the modulation crosses the 100% negative modulation   
   >line.   
      
   Yes, clearly the phase must not be allowed to flip with the signal or   
   the detector won't work. A high-Q tuned circuit will do the job   
   nicely, carrying the original phase through the inverted peak.   
      
   A PLL can also suffer the same problem, depending on the time constant   
   of the loop. The effective Q must be the same whether resonator or   
   PLL.   
      
   d   
      
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