Plural phase locked loop frequency synthesizer

Claim 1

A frequency synthesizer for generating a signal of a set frequency, comprising: variable frequency oscillating means the output frequency of which is controlled by a control signal; first frequency signal generating means for generating a first frequency signal of a set frequency; first frequency setting means connected to the first frequency signal generating means for setting the frequency of the first frequency signal; second frequency signal generating means for generating a second frequency signal of a reference frequency; a first frequency converter connected to the first and second frequency signal generating means for frequency converting the first and second frequency signals therefrom relative to each other; a first phase comparator connected to the outputs of the variable frequency oscillating means and the first frequency converter for phase comparing the outputs thereform, the compared output from the first phase comparator being applied as said control signal to the variable frequency oscillating means thereby constituting a first phase lock loop which operates so as to make the output frequency of the variable frequency oscillating means coincide with the output frequency of the first frequency converter, and the first phase comparator being capable of performing the phase comparing operation even if the frequency difference between the two inputs thereto is relatively large, wherein an enlarged capture range of the first phase lock loop is provided; a second frequency converter connected to the output of (i) a selected one of the variable frequency oscillating means and the second frequency signal generating means and (ii) the output of the first frequency signal generating means, for frequency converting the two respective outputs thereform relative to each other; and a second comparator connected to the output of the second frequency converter and the output of the non-selected one of the variable frequency oscillating means and the second frequency signal generating means, for phase comparing the outputs therefrom, the output from the second phase comparator being applied as a control signal to the variable frequency oscillating means, thereby constituting a second phase lock loop which operates so as to make the frequencies of the two inputs to the second phase comparator coincide with each other, the second phase comparator being capable of performing the phase comparing operation with a narrower range of the frequency difference between the two inputs thereto as compared with the frequency difference between the two inputs to the first phase comparator, wherein the second phase lock loop is narrower in its capture range but larger in its loop gain than the first phase lock loop.