Microwave frequency division by phase locked loops

Abstract

It is known to generate mircowave frequencies by locking the fundamental output frequency of a microwave oscillator to a multiple or sub-multiple of the output frequency of a highly stable crystal oscillator operating at a lower frequency than the microwave oscillator. Phase-locked loops are used together with frequency multipliers or dividers. One problem is that the frequency lock range decreases as successively higher and weaker harmonics are used for locking. This limits the practical harmonic locking range to multiples of approximately less than ten. The present invention eliminates this problem and allows microwave signals which have a frequency greater than 10 times the reference frequency to be phase locked without an impractical reduction of the capture range. As well, in conventional phase locked loop systems which use multipliers, fm modulation which is intentionally injected must have a modulation index less than 1.0. The present invention eliminates this problem and increases the allowed modulation index by a factor N, where N is the division ratio of the frequency divider used in the feedback loop. The present invention uses a microwave parametric frequency divider to phase lock a microwave oscillator to a stable reference oscillator without the need for frequency multipliers. The output f1 of an rf oscillator is divided by N and fed to one input of a phase detector. The phase detector has another input fed by a reference oscillator of predetermined frequency f.sub.2. The phase detector has an output fed to a control input of the rf oscillator which causes the rf oscillator to produce a frequency of Nf.sub.2 which is directly fed to the rf output of the apparatus. Note that no multiplier is required.