Abstract

A low noise frequency synthesizer 10 that uses uses frequency dividers 13, 15, at least one of which(divider 15) is incrementable and decrementable in half integer steps, and analog gain compensation in a phase/frequency detector 14 to achieve lower noise, lower spurious levels and faster switching speed than traditional methods of frequency synthesis. The key features of the present invention are its half integer dividers 13, 15 and the ability to adjust the phase detector gain to compensate the loop for varying divide numbers. The synthesizer 10 comprises two dividers 13, 15 that provide two reference frequency signals that are a function of an input signal and an output signal of the synthesizer 10. A voltage controlled oscillator (VCO) 18 provides the output signal (fO) of the frequency synthesizer 10. A phase/frequency detector 14 compares the reference frequency signals and provides a phase error output signal that drives the VCO 18. A digital controller, comprising a PROM 11 and a digital to analog converter 12, adjusts the gain of the phase detector 14 to compensate for varying divide numbers, and selects the divide numbers. The synthesizer 10 has lower phase noise, lower spurious levels and faster frequency channel switching. To generate a frequency step of a given size, the divide number is half the step size. With a divide number of half the step size, the noise contribution from the divider is reduced by that much. The phase margin is improved with a corresponding reduction in loop peaking in the noise spectrum. A missile radar system employing the present synthesizer is also disclosed.