RF Measurement Concepts

For the characterization of components, systems and signals in the radiofrequency (RF) and microwave ranges, several dedicated instruments are in use. In this article the fundamentals of the RF signal techniques are discussed. The key element in these front ends is the Schottky diode which can be used either as a RF mixer or as a single sampler. The spectrum analyser has become an absolutely indispensable tool for RF signal analysis. Here the front end is the RF mixer as the RF section of modern spectrum analyses has a ra ther complex architecture. The reasons for this complexity and certain working principles as well as limitations are discussed. In addition, an overview of the development of scalar and vector signal analysers is given. For the determination of the noise temperature of a one-port and the noise figure of a two-port, basic concepts and relations are shown as well as a brief discussion of commonly used noise-measurement techniques. In a further part of this article the operating principles of network analysers are shown. A distinction can be made between scalar and vector network analysers and their methods of measuring the transmission or reflection coefficients are explained. As digital signal processing has become cheap and easily available over the last 30 years, these instruments have become extremely versatile and powerful. Fourier transformation permits time-domain measurements and allows the removal of undesired parts of the signal trace in the time domain by gating. Network analysers require sophisticated calibration procedures, which are now indispensable for many measurement applications. Non-linear network analysis completes tests of most characteristic amplifier features. The Smith chart is a very valuable and important tool that facilitates interpretation of S-parameter measurements. The last part of this article gives a brief overview of how to use the chart. Its definition as well as an introduction on how to navigate inside the chart are illustrated. Useful examples show the broad possibilities for use of the chart in a variety of applications. This article is a compilation of previous CAS proceedings where information being in the scope of the intermediate-level RF course was extracted.