Saw-oscillator-modulator

Description: Adding a diode, a resistor, and a capacitor to the surface-acoustic-wave (SAW) oscillator enables its use in frequency-shift-keying (FSK) applications. The diode (D1), resistor (R1), and capacitor (C1) create a simple diode switch where D1 shunts C1 to ground. When the digital FSK input at R1 is low, D1 is off, allowing the small junction capacitance of D1 to couple C1 to ground. A high FSK signal causes current to flow through R1 and D1. The dynamic impedance of D1 is low during forward conduction, providing C1 with a lower impedance path to ground. Consequently, the FSK input effectively switches C1 in and out of the oscillator's circuit. When C1 is included, and the digital FSK signal is high, it slightly lowers the frequency of the circuit due to the additional phase shift introduced by C1 at the gate terminal of the GaAs FET. The SAW device limits the frequency shift to typically less than 20 ppm for a high-Q SAW device. The oscillator operates at a center frequency of 896.2 MHz with an FSK deviation of 17 kHz when driven by a 0 to 5 V signal. The frequency is also influenced by inductance (L1) and capacitance (C2).

In this circuit configuration, the integration of a diode, a resistor, and a capacitor into the SAW oscillator serves a crucial role in enabling frequency modulation through frequency-shift-keying. The diode (D1) acts as a switch that can effectively couple the capacitor (C1) to ground depending on the state of the FSK input signal applied at resistor (R1). When the FSK signal is low, the diode is reverse-biased, and the capacitor is effectively disconnected from the circuit, allowing the oscillator to maintain its original frequency.

Conversely, when the FSK signal is high, the diode becomes forward-biased, allowing current to flow through R1 and D1. This action connects C1 to ground, thereby altering the circuit's characteristics. The dynamic impedance of D1 in the forward conduction state is low, which allows C1 to influence the oscillator circuit by providing a path of lower impedance to ground. This results in a slight frequency shift due to the phase shift introduced by C1 at the gate of the GaAs FET, which is a critical component in maintaining the oscillator's performance.

The frequency-shift capability is limited by the characteristics of the SAW device, which is designed to operate with high quality (Q) factors. The frequency deviation achieved in this configuration is specified at 17 kHz, with the center frequency of the oscillator being 896.2 MHz. This performance is contingent upon the values of the inductor (L1) and the additional capacitor (C2) in the circuit, which also play significant roles in determining the overall frequency response and stability of the oscillator. The design and component selection must ensure that the oscillator maintains a high level of performance while accommodating the modulation requirements of FSK applications.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.