selective CW filter

Certain requirements must be met when designing CW filters (CW = carrier wave — basically, unmodulated morse telegraphy). The response curve must be sufficiently narrow to permit as little QRM as possible, but must be wide enough to overcome receiver drift. It must also be phase linear within the bandwidth. Readily available filters usually have a bandwidth in the order of 500 Hz (6 dB points) and give approximately 60 dB attenuation at around 1 to 2 kHz. In many instances CW signals are insufficiently filtered, so that ringing can often occur in the receiver. A filter with a very high Q allows for maximum selectivity, but is, of course, totally unsuitable for this application. The filter must also be easily reproducable, without any trimming if possible, and the cost must be kept to a minimum. The circuit shown here fulfils all these requirements and has a corn- paratively low centre frequency thereby reducing any effect that component tolerances would have at higher frequencie...