Abstract: A band pass filter multiplies an input waveform with first trigonometric functions at a plurality of frequencies and then filters the products. Furthermore, with that filtering step, the products are multiplied, or their phase is changed by a predetermined value, and the results are multiplied by the inverse of the trigonometric functions.The sum of this, for all frequencies, generated a filtered output waveform. Where the input waveform has two components, the corresponding outputs can be obtained from the read and imaginary components of the output waveform. In practice the trigonometric functions are produced by sine and cosine components at each frequency. In this way a band pass filter device is formed which is suitable for use in the control system of an apparatus which must be controlled continuously, such as a control system for a rotor.