a local oscillator calibrator comprises a main charge pump that drives a voltage controlled oscillator through a loop filter . a second , replica charge pump can also drive the vco , but is setup to output only its most positive or most negative analog output control voltage . since the construction and characteristics of the replica charge pump duplicate the main charge pump , the main charge pump &# 39 ; s minimum and maximum analog control outputs can be cloned out to the vco on demand . a vco calibration procedure therefore includes switching the vco to each of its ranges set by a bank of fixed capacitors , and using the replica charge pump to drive the vco to its minimum and maximum frequency for each range setting . the min - max frequency data is stored in a lookup table , and operational requests to switch to a new channel frequency can be supported with a priori information about which fixed - capacitor range selection will be best .