Abstract: A method of providing DC coupled frequency modulation without center frequency offset of a RF carrier frequency generated in a phase-locked loop circuit. The circuit is automatically calibrated to generate a feedback signal for cancelling DC offsets at the input of the coupling integrator with a digital-to-analog converter and resistor network driven by a digital counter. After removing frequency modulation, the coupling integrator's output will ramp while the offsets remain uncancelled. Incrementing or decrementing the digital counter in response to the integrator's output will adjust the feedback signal until the offsets are cancelled and the integrator ceases to ramp. The counter value is stored in a memory to allow subsequent presetting of the feedback signal to the calibrated magnitude. Holding the feedback signal constant thereafter during DC coupled frequency modulation keeps the RF carrier at center frequency.

Abstract: Apparatus to provide calibrated frequency modulation (FM) of frequencies generated in a phase-locked loop is disclosed. A phase-locked loop (PLL) includes a voltage-controlled oscillator (VCO) which is locked to a loop frequence signal frequency. A modulo-10 divider is included which adds integral multiples of two pi radians of phase to the output signal of the VCO whenever the modulus of the divider is reduced by an integer. This forces the PLL to step change the loop frequency by an integral multiple of the reference frequency. A sampling circuit senses and measures the change in the VCO control voltage resulting from the step 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) to adjust the gain of the DAC.

Abstract: Apparatus is provided to frequency modulate (FM) RF carrier frequencies generated in a phase-locked loop (PLL). FM outside the loop bandwidth is AC coupled to the loop. FM within the loop bandwidth is DC coupled via an integrating OP-AMP to provide phase modulation. Whenever the output of the integrator exceeds a preset threshold, a prescaler removes or adds two pi (or integral multiples of two pi) radians of phase change from the input to the loop divide-by-N circuit. At the same time, a current source pumps charge into the integrator to exactly compensate for the amount of phase removed or added resetting the integrator. DC feedback around the integrating OP-AMP and a feedback signal proportional to the exact amount of phase added or removed provides a true synthesized loop center frequency with no drift. When the feedback path is interrupted, DC FM is provided.