XNOR gates can act as digital frequency mixers. When one input is high, the output matches the input. When one input is low, the output inverts the input. This, in a sense, multiplies the two input signals, giving the sum and difference of the two input frequencies. In this example, a 1000kHz signal is being multiplied (mixed) by a 800kHz signal, producing a new signal with a 200kHz difference frequency and a 1800kHz sum frequency.
