Adding a diode, a resistor, and a capacitor to the SAW (surface-acoustic-wave) oscillator allows you to use the oscillator in FSK (frequency-shift-keying) applications. Dl, Rl, and Cl form a simple diode switch in which Dl shunts Cl to ground. When the digital FSK input to Rl is low, Dl is off, and the small junction capacitance of Dl couples Cl to ground. A high FSK signai causes current to flow through Rl and Dl. Dl `s dynamic impedance is small when it is in forward conduction. Therefore, Cl sees a lower impedance path to ground.
Thus, the FSK input effectively switches Cl in and out of the oscillator"s circuit. When Cl is in the circuit-digital FSK is high-it pulls the frequency of the circuit slightly lower because of the additional phase shift Cl introduces at the GaGs FET gate terminal (available from: Dexcel, Div. of Gould, Santa Clara, CA). The SAW device (available from: RF Monolithics, Dallas, TX) restricts the amount of frequency shifting-usually less than 20 ppm for a high-Q SAW device. The oscillator shown produces a center frequency of 896.2 MHz with an FSK deviation of 17 kHz when you drive the FSK input with a 0 to 5 V signal. The frequency also depends on L1 and C2.