This circuit uses the analog multiplication properties of the XNOR gate to multiply two input signals, and thus add and subtract their frequencies, with the lower frequency being let through by a lowpass filter (to block the higher frequency and the harmonics), and the higher frequency being let through by a band pass filter (to block the lower frequency and the many harmonics).
The top two colors (blue and green) are the inputs, the lower of the middle colors (orange) is the sum of the two frequencies of the inputs, and the bottom color (red) is the difference in the frequencies of the two inputs.
