High Selectivity with Constant Phase over the Pass Band

The frequency dependence of the phase in selective circuits causes problems in systems that utilize the signal phase as a carrier of information. For example, in a Doppler system the phase change originating within the receiver represents an error in the measurement. A relatively simple method for compensation of phase shifts in filters, resulting in a constant output signal phase, has been developed. In principle, the signal frequency is inverted by means of a mixer, and the signal is passed through a filter to create a phase shift in one direction. The signal frequency is then reinverted in another mixer, and the resulting signal is again passed through a filter which now effects a phase change in the opposite direction. By properly choosing the filter constants and combining the two signals, the two phase shifts will compensate each other, giving an output signal whose phase is independent of the frequency. The method of phase compensation is not restricted to a particular frequency range or a particular type of circuitry. What is needed in addition to conventional circuitry without phase compensation is a local oscillator and simple mixers. In some applications, a local oscillator is provided already (for example, parametric amplifiers or frequency converters), making the adaptation of the compensation principle very simple and economical.