The original criteria for a feedback oscillator, credited to Barkhausen, establishes that two conditions must be met in order for a circuit to sustain oscillations. The first is that the gain around the loop be greater than unity, and the second is that the loop phase be an integer multiple of 360.degree.. The two major types of devices used in these applications are delay lines and resonators. Surface acoustic wave devices have been used in feedback radio frequency oscillators as delay lines as shown in U.S. Pat. No. 3,868,595 of Capps et al. A principal disadvantage of the delay line is that a fixed phase slope is established during the design of the device by the physical spacing between the input and output transducers. The phase slope has a reciprocal correspondence with the physical spacing which is to say that for a delay line to have a steep phase slope, the device would need to be extremely long. The phase slope determined by the physical spacing is a fixed parameter in the design of the oscillator and establishes the oscillator frequency at the point along this phase slope where the loop phase criteria are met. In order to shift or pull this frequency of oscillation from the established point, additional phase shift elements must be added to the circuit to force the oscillation to occur at a different point on the phase slope while maintaining the original integer multiple of 360.degree. loop requirement.
A SAW resonator includes one or more transducers within a resonant cavity formed by distributed SAW reflecting arrays. The reflection coefficient of these arrays can readily exceed 99 percent, and the electrical Q of the resonator is determined by dissipative losses, either within the device or in external circuitry. A resonant cavity exhibits a 90.degree. phase shift between the three dB bandwidth determined from the Q. The higher the device Q, therefore, the steeper is the phase slope.
Resonator oscillators are also well known in the art. Bulk mode crystals are used from a few kilohertz to less than 21 megahertz fundamental frequency and can have third to fifteenth overtones giving a 300 megahertz frequency. A free running oscillator can be phase locked to a lower frequency. SAW resonator oscillators are also known which use impedance transparent phase shifters as shown in 1976 Ultrasonics Symposium Proceedings, IEEE Cat. #76 CH 1120-5SU "Voltage Controlled Surface Wave Resonator Oscillators" by Lawrence Ragan.
USPTO Backgrounds
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