state variable filter

7-32 elektor july/august 1980 0 15 O P1 = ()factor preset P2 = fo preset C611000 80525 Many multi-band receivers and cheap communications receivers have a bandwidth compatible with the popular broadcasting stations which is too wide for amateur radio purposes. Narrow band receivers or — even better — receivers with an adjustable bandwidth are usually only found in more expensive systems. In order to be able to listen to amateur radio transmissions (SSB and CW) without too much inter- ference, on a broad band receiver, this ingenious audio band filter may provide a solution. It is a state variable filter, that is one in which both the centre frequency and the bandwidth can be varied. If the filter is placed before the existing AF amplifier, it will be possible to attenuate any interference signals by tuning the filter as accurately as possible to the frequency of the received audio signal. It is of course no comparison to a "real" narrow band receiver, but the result is usually...