Filter circuit arrangement

A filter circuit (2) is automatically tuned so that the positions of the poles and zeros of its frequency response characteristic are maintained in substantially fixed ratios to an input frequency (fr). To this end the arrangement comprises a frequency discriminator (1) having its output 5 coupled to a controllable element (8) of a resonant circuit included therein, so that the resonant frequency of the resonant circuit is maintained equal to the input frequency. The output 5 also controls a counterpart (13) to element 8, which counterpart is included in an auxiliary circuit (3). The filter circuit is tuned by controlling an element (10) included therein, which element also has a counterpart (14) included in the auxiliary circuit, both these last elements being controlled by the auxiliary circuit output signal. The auxiliary circuit output signal is determined by the ratio between parameters of the respective counterpart elements (13,14) which correspond to parameters of the tuning elements (8,10) which govern the tuning frequencies, and acts to maintain this ratio, and hence the aforementioned ratios, substantially constant.