Abstract: A frequency down-converter down converts an input signal having a frequency in a given bandwidth and a frequency up-converter up converts the down converted input signal and imparts a doppler shift to the up converted signal to generate an output signal having a frequency with doppler shift in the given bandwidth.

Abstract: This invention relates to a gated frequency difference detector for use in a frequency lock loop. The gated frequency difference detector consists of a dual output hybrid quadrature mixer, two parallel video signal processing channels, a digital phase/frequency detector, a digital scaler and a digital-to-analog converter. The difference in frequency between the RF input and the output of a local oscillator is measured. Two orthogonal frequency difference outputs are first generated, one of which either leads or lags the other depending on the sense of the difference. Signals are then bandwidth limited and amplified in two video channels and converted to logic levels. The phase and frequency are finally digitally detected and processed to generate a bipolar voltage proportional to the magnitude and sense of the frequency difference.