Importance

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Abstract

A frequency synthesizer includes a control circuit which outputs changeover signals for bringing one of a number of filters included in a loop filter into electrical connection with a VCO outputting a desired frequency signal, such that a phase difference signal passes through one filter when a demand for frequency changeover is made, and provides a frequency division ratio corresponding to said one filter to a variable frequency divider and a reference frequency divider whose phases are compared to produce the phase difference signal. In addition, the control circuit successively supplies changeover signals for performing selection of respective filters through each of which the phase difference signal passes, and provides frequency division ratios corresponding to filters tuned to the variable frequency divider and the reference frequency divider to change the output frequency of the VCO incrementally. Furthermore, the control circuit provides changeover signals for connection filters employed in a steady state. The time required between each step in the frequency changeover process can be shortened by setting a time constant for each of the filters to a value such that an intermediate output signal representative of a frequency determined by the frequency division ratio corresponding to each filter is stabilized in a short period of time.

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First Claim