From: apogosso@tpg.com.au   
      
   "John Byrns"  wrote in message   
   news:byrnsj-CD7991.17060917072011@news.giganews.com...   
   > 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.   
   >   
   > Adding a phase detector and a VCO to your circuit will fix this problem.   
      
   Absolutely correct! One can make a detector consisting of a high-speed   
   comparator (MAX941 is a good choice) and a CMOS switch (74HC4066 or even   
   74LVC1G66), and it would work down to very low levels of signal, to about   
   2mV, but it will never "properly" demodulate suppressed carrier signals with   
   modulation >100%. Phase reversals in the signal immediately trannslate into   
   phase reversals on the comparator output.   
      
   > 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?   
      
   Alex:   
   No transmitter will do that, but a carrier might lose its amplitude due to:   
   - fading;   
   - multi-path propagation (often in the vicinity of some large objects like   
   street wiring, transmission lines, etc.);   
   - double peak tuning of the IFT.   
      
   >   
   > --   
   > Regards,   
   >   
   > John Byrns   
   >   
   > Surf my web pages at,  http://fmamradios.com/   
      
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