Razor-sharp CW filter

The circuit consists of four stages of active bandpass filtering provided by two type-µ integrated-circuit dual op amps and includes a simple threshold detector (diodes Dl and D2) between stages 2 and 3 to reduce low-level background noise. Each of the four filter stages acts as a narrow bandpass filter with an audio bandpass centered at 750 Hz.
Razor-sharp CW filter - schematic

The actual measured 3-dB bandwidth is only 80 Hz wide.

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