8.1 Introduction

Frequencies usually designated as being in the microwave range cover 1 to 30 GHz. The lower boundary corresponds approximately to the frequency above which lumped-element modeling is no longer adequate for most designs. The range above is commonly referred to as the millimeter range because wavelengths are less than 1 cm, and it extends up to frequencies where the small wavelengths compared with practically achievable phyical dimensions require quasioptical techniques to be used for transmission and for component design. The emphasis of the following discussion will be on factors that affect the design and operation of signal sources in the microwave frequency range, though many of the characteristics to be discussed do apply to the neighboring ranges as well. Methods for the generation of signals at lower frequencies employing synthesis techniques are also described, since up-conversion can be performed readily to translate them up into the microwave and millimeter ranges.

Application for such sources include use in microwave signal generators (see Chap. 18), as local oscillators in receivers and down-convertors, and as exciters for transmitters used in radar, communications, or telemetry. The tradeoffs between tuning range, spectral purity, power output, etc. are determined by the application.

Previous generations of microwave sources were designed around tubes such as the Klystron and the backward-wave oscillator. These designs were bulky, required unwieldy voltages and currents, and were subject to drift with environmental variations. More recently, compact solid-state oscillators employing field-effect transistors (FET) or bipolar...