Image Number 2 for United States Patent #4885801.
A mixer mixes received frequency modulated signals with a frequency provided by a voltage controlled oscillator to generate intermediate frequency signals. The intermediate frequency signals are filtered by a filter and input to a demodulator to generate low frequency signals. A control voltage is derived from the demodulator to control the resonant frequency of the filter in response to the frequency shift of the intermediate frequency signals. A phase rotator is coupled between the demodulator and the oscillator to phase rotate the control voltage of the oscillator. In a preferred embodiment, the phase rotator is an inverter.