Automatic frequency control for radar receiver

Abstract

A digital automatic frequency control for a pulse-type radar receiver having a voltage controlled local oscillator. A first digital number is generated and converted to a first analog voltage for coarse control of the local oscillator frequency. The first digital number is varied during the time output from the radar transmitter is present at the receiver mixer. The first digital number is varied causing sweep of the local oscillator frequency. When the difference frequency output from the mixer is at a value to cause output from the receiver intermediate frequency amplifier of a predetermined level, variation of the first digital number is halted, providing a coarsely tuned local oscillator frequency. A second digital number is then generated, converted to a second analog voltage which is combined with the first analog voltage. The second digital number is varied to cause the local oscillator frequency to be sidestepped above and below the coarsely tuned frequency. Samples of the receiver output are separately accumulated for sidesteps above and below the coarsely tuned frequency. The difference between the separated accumulated outputs is used to generate a third digital number which is converted to a third analog voltage. The third analog voltage is combined with the first analog voltage to fine tune the local oscillator frequency.