5.1: Introduction

Most RF oscillators generate sinusoidal signals that are either used to drive mixers or, if modulated, to produce frequency modulated signals directly. In some designs the microwave oscillators drive flip-flop circuits that produce periodic square-wave signals with multiple phases as required for the LO drives of I/Q modulators. RF and microwave oscillators can be designed using either a two-port or a one-port approach. The classic treatment of oscillators is based on a two-port gain device with a feedback loop, but the oscillator can nearly always be viewed, and thus more conveniently designed at microwave frequencies, as a one-port in which a resonant circuit, called the tank circuit, is connected to an active circuit that presents a negative resistance. However, the stability, noise, and start-up analyses of an oscillator are based on a two-port with feedback.

All microwave oscillator designs are based on one of three basic oscillator configurations. Suitable configurations must have as few reactive elements as possible while enabling stable single-frequency oscillation. These configurations have three or four reactive element. Mapping a microwave oscillator design on to one of the standard oscillator designs is not simple mainly because active devices at microwave frequencies have substantial parasitic capacitances and also adjustments must be made to accommodate biasing. Stability of a microwave oscillator is of great concern and the essence is that there should be as few significant energy storage elements as possible. If there are parasitic energy storage elements these should be quite small or absorbed into the capacitors of one of the basic oscillator configurations. With more energy storage elements than necessary the chances of unwanted resonances is much higher and hence instability of an oscillator is more likely.

At microwave frequencies the \(Q\) of lumped elements is limited. A lumped inductor in the tank circuit has a particularly low \(Q\) and if there is room it is replaced by a transmission line. Oscillators that are fixed in frequency can use a resonant circuit with high \(Q\) circuit elements. This contrasts with voltage-controlled oscillators (VCOs) that have a lossy variable element, nearly always a varactor, in the tank circuit in what is now a low \(Q\) resonant circuit. The voltage-controlled variation of (invariably) the capacitance of this element changes the frequency of the oscillator.

A microwave oscillator could be realized on-chip or realized as a hybrid circuit with a packaged active device and packaged lumped-elements and possibly transmission lines for the resonant circuit. A hybrid design has much more flexibility than an on-chip design, and if designed correctly has better performance than a monolithically integrated design. Hybrid design techniques are much more mature than chip-based designs but over time some of the techniques used with hybrid designs will migrate to on-chip designs. Also, on-chip designs are preferably differential with transistors in a push-pull, i.e. differential, configuration. Better performance of an on-chip oscillator can be obtained by using an off-chip resonator.

Oscillator theory derives from the analysis of a two-port with gain and feedback. This theory is described in Section 5.2. The following sections explore practical oscillator configurations. Section 5.3 presents the design technique for designing a fixed-frequency microwave oscillator and this is followed up with a design case study in Section 5.4. The distinguishing feature here is that the resonant circuit consists of high \(Q\) elements. Section 5.5 presents a design approach for a voltage-controlled microwave oscillator. A state-of-the-art case study of a \(5\text{ GHz}\) VCO design is presented in Section 5.6. Section 5.7 describes the design of an on-chip differential oscillator. The last two sections, Sections 5.8 and 5.9 in this chapter describe oscillator phase noise, a characteristic that is the fundamental performance limiting parameter of an oscillator as, for example, it affects the sensitivity and performance of communication systems and the range of radar.