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.