A voltage-variable capacitance tuning diode is placed in series with the crystal feedback path. Changing the voltage on VR varies the luning diode capacitance and tunes the oscillator. The 510-KO resistor, R1, establishes a reference voltage for VR -ground is used in this example. A 100-KO resistor, R2, isolates the tuning voltage from the feedback loop and 0.1-I`F 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 D 1.
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 measured deviation in parts per million for several tested crystals.