RF performance optimization of a SAW based transmitter for remote keyless entry system

In recent years, SAW resonator based oscillators have found wide applications in Remote Keyless Entry (RKE) systems in automobiles. In the present paper, we present a methodology that optimizes the radiofrequency (RF) performance of a hand-held microprocessor controlled RKE transmitter to meet our objective of 10 meter minimum range. We measured the S-parameters of several SAW resonators on an HP8753C network analyzer, and used these data in Libra, a nonlinear RF circuit simulation tool. Using the harmonic balance method we modeled the SAW resonator, and extracted the equivalent circuit elements by using the optimization feature of this simulator. We then analyzed the oscillator circuit, and optimized it for stable oscillation at 315 MHz with a start-up time of <25 ns. This modeling was also verified in Pspice. We used the tolerance analysis feature of Libra to account for component tolerances to make the circuit realizable with commonly available components. The circuit was fabricated, and tested over temperature. It has met all the specifications, and FCC Part 15 requirements