Frequency range gate closure

Abstract

A frequency range gate closure circuit for use in synthetic aperture radars that incorporate a digital waveform generator or direct digital synthesizer that improves the mapping resolution of the radars. Range gate closure motion compensation is more accurately implemented using the present circuit. The digital waveform generator or direct digital synthesizer is clocked by a system clock, and processes control words corresponding to a desired slope of the stretch frequency modulation of transmitted radar signals, to generate frequency modulated pulse output signals. The range gate closure circuit includes a digital data accumulator for receiving an increment value and a pulse repetition frequency value of the radar. Logic circuitry is coupled to an output of the digital data accumulator and is coupled to receive a digital modulo threshold signal and output the increment value when it is less than the digital modulo threshold signal every pulse repetition interval. The difference between the digital modulo threshold signal and the increment value is coupled to the digital data accumulator to reset it when the increment value is less than the digital modulo threshold signal. A coarse time delay counter is coupled to the logic circuitry for counting desired coarse time delays needed by the circuit. The frequency range gate closure circuit decrements a frequency offset of a local oscillator frequency sweep by the increment value every pulse repetition interval until the frequency offset is less than the digital modulo threshold signal so that the frequency offset is equal to the expected Doppler frequency error.