From: apogosso@tpg.com.au   
      
   "Alex Pogossov"  wrote in message   
   news:4e22cf2a$1@dnews.tpgi.com.au...   
   >   
   > "Don Pearce"  wrote in message   
   > news:4e22c684.16025155@news.eternal-september.org...   
   >> On Sun, 17 Jul 2011 04:14:48 -0700 (PDT), Patrick Turner   
   >>  wrote:   
   >>   
   >> 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).   
   >   
   > Alex:   
   > I have with success. In fact I fit a synchronous PLL based detector module   
   > into old radios. It allows to detune the radio so that a carrier sits on   
   > the slope of the IF responce curve, suppressed and working only as a pilot   
   > tone, so to speak. Demodulation is performed by CMOS switches. Virtually   
   > only one sideband is used. This sideband now almost completely falls into   
   > a 6...7kHz IF bandwidth. The sound is amazing -- like on FM, and even   
   > better because there is no this "screeching" sometimes caused by multipath   
   > propagation. By detuning the radio one way or another you can choose a   
   > sideband less jammed by interference.   
   >   
   > On crowded SW it is still better not to detune the radio, but uave the   
   > carrier in the middle. It gives more loud reception of weaker stations.   
   > Resilience to fading is great. SW becomes listenable. For fun I listen to   
   > "Radio Russia" in Russian and also to Chinese propaganda intended for   
   > Russia (also in Russian).   
   >   
   > Unfortunately for the tubefanatics on this list, this synchro module CAN   
   > NOT BE BUILT using TUBES. It uses ICs: TLE2704, 74HC74, 74HC4046, MCP602,   
   > LM339, LM78M05 and several p-JFETs in auto PLL lock band control.   
      
   I forgot the most important -- the switches DG445. Two are used for   
   quadrature demodulation, and the other two for automatic switching between   
   synchro and ordinary AM when PLL unlocks. To switch this module on and off I   
   use a pot with shaft push-pull switch (like in the guitar stuff). I fit it   
   instead of the tone control. Thus the "original" look of the radio is   
   preserved, but synchro detector is added.   
      
   Also forgot to mention that this synchro module has a delayed amplified AGC   
   (integrator) which equalises the levels of all the stations, no matter weak   
   or strong. Apart from anything else, it is important for having PLL   
   bandwidth stable.   
      
   In fact I strip the crappy booooring AM detector / AGC stuff from a   
   boooooring radio (what sits around 6AV6) and replace it with the module.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)