Abstract: A fractional divider which functions as a prescaler in a phase lock loop-based frequency synthesizer provides a selectable prescaling factor to the divide-by-N programmable divider. The prescaling factor can assume either the value 2 or 2+1/N. The fractional division, occurring in front of the programmable divider, permits a periodic signal to be generated by that device, while recapturing the frequencies lost to prescaling. When operating in the divide-by-(2+1/N) mode, the fractional divider drops one input clock pulse each time the output signal of the programmable divider assumes a predetermined binary state.

Abstract: Prepositioning of a phase lock loop by charging or discharging the loop filter capacitor to a voltage close to the desired VCO control voltage is accomplished rapidly by the addition of a current pump. The current pump, responsive to a tuning voltage corresponding to the VCO control voltage provides a pulse of high rate current to charge or discharge the capacitor. The current pulse, which is of sufficient length to ensure that the capacitor achieves the desired voltage, swamps out the contribution of the loop charge pump which is responsive to the loop phase detector. When disabled, the current pump appears as a virtual open circuit to the loop capacitor, so that no current leakage occurs through the current pump.