The heartbeat of the FLASH optical synchronization system, the Master Laser Oscillator (MLO), must be synchronized with the Master RF Oscillator (MO) via a circuit that converts the timing information in the pulses of the laser into an RF signal that can be phase-locked to the RF output of the MO. The Phase Lock Loop (PLL) is closed via a Digital Signal Processor (DSP) and has 77 fs out-of-loop drift performance over a 24 hour period with no disturbances in the climate controlled room. With amplitude drift stabilization of the laser and temperature control of the phase measurement chassis, the largest contributors to the drift of the PLL would be removed. With active temperature control of the RF phase measurement, an over-night stability of 5 fs was achieved. The design of the RF-circuit and its performance are presented here.