5.4 Oscillators

Oscillators convert dc power to ac power and are the source of RF, microwave, and millimeter-wave signals that are transmitted, received, and manipulated by radar and communication systems. They provide the ac signal at the specified frequency, which is then amplified, modulated, and transmitted through an antenna; they also provide the signal known as the local oscillator (LO), which is used to down-convert the received signals to lower frequencies, where they can be decoded easily. The important characteristics of oscillators are their frequency, frequency stability, frequency tuning and ability to lock the frequency to a reference frequency, amplitude, quality of oscillation, and phase noise.

MMIC oscillators can be made over the full RF, microwave, and millimeter-wave frequency bands, in fact, wherever the MMIC transistors have gain. They tend to be limited to low output-power levels (a few milliwatts) due to thermal and breakdown characteristics and usually require buffer amplifiers and power amplifiers at their output. These oscillators are often designed as voltage-controlled oscillators (VCOs), which allows their oscillation frequency to be tuned by an external control voltage. This ability also enables the oscillation frequency to be locked to an external reference signal so that its characteristics are improved. Many of the MMIC oscillator's characteristics are dominated by the type of resonator that is used; this is described in more detail in the following sections.