A voltage-variable capacitance tuning diode is placed in series with the crystal feedback path. Changing the voltage on VR varies the tuning diode capacitance and tunes the oscillator. The 510-KΩ resistor, R1, establishes a reference voltage for VR-ground is used'm this example. A 100-KΩ resistor, R2, isolates the tuning voltage from the feedback loop and 0.1-μF capacitor C2 provides ac coupling to the tuning diode. The circuit operates over a tuning range of 0 to 25 V. It is possible to change the tuning range from 0 to 25 V by reversing the tuning diode Dl. Center frequency is set with the 2 -60 pf trimmer capacitor.Deviation on either side of center is a function of the crystal frequency. The table in Fig. 21-7 shows mea-sured deviation in parts per million for several tested crystals.
Required Field 
