WARNING! - There is a problem with the D-Latch circuit!!
Accuracy is not good - the error can be as great as one HF clock period, which can be significant if using low frequency HF clocks or high difference frequencies!
I wrote this based on some SIMPLE simulations I have done..
It describes using an XOR gate for simple heterodyning, and using a D-Latch for clean heterodyning suited to digital circuits.
Contains schematics and resulting waveforms.
MUCH MORE ON THIS AND WAVE-SHAPING can be found Here -> http://www.element14.com/community/thread/18199