Abstract: An automatic frequency control circuit having the ability to resolve the frequency of an incoming signal to an extreme level of accuracy and to tolerate amplitude and pulse modulation on the incoming signal is disclosed. Two negative feedback loops are utilized. A coarse adjustment feedback loop resolves the frequency of the incoming signal to within the authority of a fine adjustment feedback loop. A computer is a common element of both loops and makes decisions regarding whether to resolve incoming a signal frequency using the coarse or the fine adjustment loops. Phase differences between a signal derived from the incoming signal and a reference signal are sampled at controllable time intervals apart to determine frequency adjustments within the fine adjustment loop.

Abstract: A variable frequency oscillator including a nonlinear element for varying the frequency has alternately attached thereto a phase lock loop for maintaining the base or starting frequency of the oscillator substantially constant, and a sweep generator with a rate error loop for sensing the frequency linearity of the output sweep and altering the input signal to maintain the output linear.