Anti-Parallel Diode Mixer

The Polyakov Direct Conversion circuit is a bit different.  It has the switches (the diodes) turned on twice each cycle:  When the VFO voltage goes to a positive peak, this turns on one of the diodes.  When the VFO goes to a negative peak, this turns on the other diode.   So in effect the switch is being turned on TWICE each cycle.  So with the Polyakov you run the VFO at HALF the operating frequency.  For a DC receiver designed to run around 7.060 MHz, you build a VFO at around 3.53 MHz.  This has some immediate advantages.  My favorite is that it is easier to get a VFO stable at a lower frequency.  It is easier to stabilize a VFO at 3.53 MHz than it is at 7.060 MHz. 

This circuit above demonstrates a low-cost WSPR receiver design. A 14.060 MHz crystal oscillator is pulled to 14.0623 MHz, which feeds into a Polyakov harmonic mixer. This mixer operates at twice the injection frequency, generating a 28.1246 MHz signal used for WSPR reception. The antenna signal at 28 MHz is directly injected into the mixer. The audio output is amplified and filtered in two stages before reaching the PC sound card.

The receiver exhibits a minimum discernible signal (MDS) of approximately -124 dBm. AM rejection was tested by injecting a 100% AM-modulated carrier at 28 MHz. An input of -53 dBm was required for AM detection, indicating an impressive 70 dB AM rejection.