This circuit takes audio from a receiver that might have a weak CW or tone signal and uses a PLL (Ul) to recover the wea
k signal. Ul produces a low on receipt of a tone or note of frequency, determined by Rl, R7, and C2. The output of Ul (pin 8) goes low, keys tone generator U2, and produces a new tone. The circuit is useful in cleaning up CW reception in static, noise, etc.