  For the characterization of components, systems and signals in the RF and
microwave range, several dedicated instruments are in use. In this paper the
fundamentals of the RF-signal sampling technique, which has found widespread
applications in 'digital' oscilloscopes and sampling scopes, are discussed. The
key element in these front-ends is the Schottky diode which can be used either
as an 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 analysers has a rather 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. A brief discussion of commonly used noise
measurement techniques concludes the paper.
