A Continuously Adjustable Filter for Audiofrequencies
Glenn E. Tisdale · The Journal of the Acoustical Society of America · 1949
An electronic instrument has been developed to operate as a filter with cut-off frequencies continuously adjustable over the audio spectrum. The circuit consists of a low-pass and a high-pass network in the form of four-terminal impedances. These may be used singly or connected in tandem to give band pass action. A single control selects the cut-off frequency of each network over a range of 10 to 1, while a decade switch extends the over-all range to 100 to 1, or more. The low pass section provides a pass band flat within 1 db below the cut-off frequency. Attenuation past cut-off is at the rate of 18 db per octave down to the noise level of the circuit. The high-pass section provides a pass band flat within 2 db above the cut-off frequency. Suppression is more than 32 db for the first octave below cut-off, and the attenuation continues to increase down to the noise level. Noise is at least 60 db below a signal of one volt. The voltage attenuation in the pass band of the complete filter is about 6 db. The principle of operation of the instrument is based on active feedback networks using only resistance and capacitance. The variable elements are ganged resistors, whose values may be in error by several percent without affecting the performance described. Because there are no high gain stages in the circuit, distortion and noise problems are reduced to a minimum.