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Abstract

A voltage controlled oscillator circuit incorporating a closed loop coarse tuning mechanism. In this system, a reference oscillator is set with the desired frequency for the voltage controlled oscillator. A resulting voltage used to drive the oscillator is produced by a synthesizer connected in series with a loop filter. The resulting voltage is connected to a fine tune input of the voltage controlled oscillator and also to the input of an adaptive closed loop coarse tuning mechanism. The adaptive closed loop coarse tuning mechanism is comprised of an op amp configured in a noninverting feedback loop connected to a parallel resistor/adapt switch loop. The loop is followed by a shunt capacitor filter which is then connected directly to the coarse tune input of the VCO. If the adapt switch is closed, current from the coarse amp flows through a filter and to a coarse tune port of the VCO. The voltage resulting from the adaptive closed loop coarse tuning mechanism overrides any voltage being received in the fine tune port, making the coarse tune loop controlling in frequency acquisition. Throughout the entire coarse tune process, a feedback loop allows the synthesizer to adjust the control voltage as the VCO gets closer to the desired frequency. After a predetermined acquisition time, the adapt switch is opened. This virtually eliminates the adaptive closed loop tuning mechanism from the circuit because of the large time constant associated with the (closed) adapt switch/resistor circuit. At that point the fine tune port voltage will make any minor adjustments in driving the VCO to the desired frequency.

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