Two methods of oscillator analysis and design are considered in this book. One method involves the open-loop gain and phase response versus frequency. This Bode response and nonlinear effects discussed later predict many aspects of oscillator performance. A second method considers the oscillator as a one-port with a negative real impedance to which a resonator is attached. The loop method provides a more complete and intuitive analysis while the negative resistance method is more suitable for broad tuning oscillators operating above several hundred megahertz.

The phase noise performance of oscillators has become increasingly important because communications channels have become closer spaced and more heavily loaded, data transmission systems often require low phase noise, military EW and CCC systems are more sophisticated, and higher frequencies are being used by a variety of systems. Broadband voltage-controlled oscillators used in electronically tuned PLL applications, which are now common, are inherently noisy.

Historically, the analysis of oscillator circuits involved the selection of a particular topology and then finding symbolic equations for that topology. Once the symbolic equations were found, evaluating which circuit element values satisfied the oscillation criteria required little computational effort. Abundant technical literature exists with symbolic equations for various oscillator structures. This technique was appropriate in a day when numeric tools were the slide rule and paper and pencil. Unfortunately, each time the engineer wishes to investigate a new oscillator topology the symbolic equations must be found. The symbolic approach is efficient for designing oscillators of one type but oppresses exploration of alternative and therefore, perhaps, more optimum structures.

Oscillators using resonators constructed from inductors and capacitors find wide application in electronic systems. L-C oscillators are more stable than R-C oscillators for both the long and the short term. As a consequence, L-C oscillators are almost exclusively used for general oscillator applications above the lower-frequency limit for which inductors are practical in size. Economic high quality capacitors are available for a wide range of values and seldom affect oscillator design as much as inductors. At low frequencies, the value of inductance required for reasonable reactance becomes large. Cores using magnetic materials help decrease the practical lower-frequency limit.

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 highloaded-Q oscillators in the frequency range 200 to 1,200 MHz. Typical unloaded Qs are 6,000 to 12,000.

This chapter utilizes the principles presented earlier to study oscillators from the designer's perspective. For each case we begin with a specification, discuss the design qualitatively and then proceed with the actual design. Each case is an oscillator which satisfies a typical application requirement.