Introduction

This chapter is mainly concerned with the measurement of the microwave response of a circuit. The equipment that performs these measurements is discussed briefly in Section 4.1, which reviews the capabilities of the network analyser. Following this section a detailed description is given on how to extract the unloaded quality factor of a cavity or resonator from measurements taken on a network analyser. It is well known that the unloaded quality factor can be estimated by simply taking the ratio of the centre frequency to the 3 dB bandwidth of the transmission response of a resonator, and the unloaded Q can be calculated by simply using the insertion loss measured at resonance. However, although this may be adequate for many situations, greater accuracy may be required or reflection measurements from the cavity may only be available. Calculations are therefore given for both one- and two-port measurements. For the two-port case both symmetrical and asymmetrical coupling are considered. Throughout the discussion, indications of how the accuracy of quality factor measurements may be improved are given. The final section of this chapter deals with power dependent measurements. The calculation of the energy stored in a cavity or the current on a resonant transmission line from S (scattering) parameter measurements is given.

Measurement system

Modern microwave measurement equipment, although expensive, allows accurate measurements to be made with the minimum of effort.