Overview

High-Q oscillators up to 200 MHz are built using bulk quartz crystal resonators. For UHF and microwave frequencies, transmission line and cavity oscillators offer higher Q than L-C designs, but they are larger. SAW resonators fill the need for small high-loaded-Q oscillators in the frequency range 200 to 1,200 MHz. Typical unloaded Qs are 6,000 to 12,000.

SAW resonators are commercially available in both two-terminal and two-port forms. As with bulk quartz crystals, tuning is difficult. The operating frequency is set at the time of manufacture, so SAWs must be tooled and stocked for each oscillator frequency requirement.

The shunt capacitance, C _o, of SAW resonators is proportionally smaller than the parallel capacitance of bulk crystal resonators. Therefore, the SAW oscillation frequency can be pulled farther. This advantage is somewhat mitigated by the fact that the initial tolerance and stability are poorer, and often a significant portion of the pulling range is required just to "net" the oscillator frequency. Additional information on SAW resonators is given in Section 2.15. Three different oscillator configurations using SAW resonators are considered next.

The bipolar SAW oscillator uses a common-emitter amplifier configuration and a two-terminal SAW resonator. It is a cost-effective design. The hybrid and dual-gate MOSFET oscillators use two-port SAW resonators. The hybrid SAW design results in an excellent characterization of oscillator performance and is the best choice if frequency pulling is required. The dual-gate MOSFET design has had widespread application, and can be used with 0 or 180 resonators by...