[usa2003ardf] Direct conversion receiver

[Kuon & Dale Hunt]

>Direct conversion receiver...

   My receiver kits are direct conversion (as are several of
   the European circuits I've collected).  Most have VFO's,
   but I will be making some "beginner" kits for Scout groups
   and the like using crystal control for simplicity.  (One
   less control to have set wrong!)  I did a prototype a
   couple of years ago, and it worked fine.  (Just plan for
   a trim cap across the crystal in case you don't know the
   EXACT transmitter freqs.)

   The standard in the Region 1 rules is that the receiver
   can not cause interference to other competitors at a
   distance of 10m.  I think my first prototype was barely
   audible on the Altai at 25', so just within acceptible
   range.  Some additional shielding of the VFO circuitry
   helps a lot.  And a metal case makes a big difference!

   I did use a cascode amplifier with a balanced mixer - both
   help to reduce radiation from the loop itself.  So far
   it appears that most of the radiation is from the VFO
   wiring itself.

   Rainer Floesser DL5NBZ used half-frequency VFO injection in
   his "Junior 80" receiver to avoid problems with radiating
   a signal on the transmitter frequency.  Of course, that won't
   work with a 3579 crystal.


>
>I'm shooting for gain knob and nothing else.


   Sam -  one other suggestion:  you might consider using a
   4-position rotary or slide switch for the gain control
   instead of the pot.  Makes it much easier to estimate
   relative strength, since the settings are reproducible.
   I have a Swedish article that does this with a simple
   MPF102 / NE612 / LM386 receiver that has some interesting
   features, as well as some others if you want some ideas.

   (And I have ideas to get rid of the gain knob as well, but
   don't know if I will get it built by August...)

       - Dale WB6BYU