FM CALIBRATION IN A PHASE-LOCKED LOOP

Apparatus is provided for effecting calibrated frequency modulation (FM) of frequencies generated in a phase-locked loop. A phase-locked loop (PLL) includes a voltage-controlled oscillator VCO which is locked to a loop reference signal frequency. Whenever the loop frequency is changed a calibration cycle is initiated in which a modulo- 9/10 divider (22) included in the loop is operated to add integral multiples of two pi radians of phase to the output signal of the VCO. This forces the PLL to step change the loop frequency by an integral multiple of the reference frequency. A sampling circuit (20,24) senses and measures the change in the loop frequency. The change in the control voltage is a measure of the VCO gain at that frequency. The change in the control voltage is converted to a digital number which is loaded into a multiplying digital-to-analog converter (DAC) inserted in the PLL to adjust the gain of the DAC. The effective combined gain of the DAC and the VCO is constant and independent of the VCO gain over the entire range of the VCO thereby providing an equal amount of modulation from an FM signal at any frequency of the VCO.