6.1 Introduction

The majority of signal amplitude or level measurements made at RF and microwave frequencies are measurements of power rather than voltage. Voltage measuring instruments such as RF millivoltmeters are still in frequent use but are nowadays less common than in the past. Probably, the most widespread test instrument is the oscilloscope, and modern oscilloscopes have bandwidths that extend well into the RF and microwave frequency ranges. There are a variety of tests and measurements of highspeed waveforms made with oscilloscopes and similar signal analysis instruments that rely on accurately probing and capturing signals at RF and microwave frequencies. This chapter overviews a variety of RF and microwave measuring instruments, including digitising and sampling oscilloscopes. It briefly describes their operating principles and characteristics, discusses probing and loading issues, and considers traceability and calibration for these instruments.

Figure 6.1 is a chart illustrating part of the electromagnetic spectrum and a number of typical applications. The range of the electromagnetic spectrum described by the term 'RF' (meaning radio frequency) is usually considered to begin at the frequency where AM broadcast radio transmission takes place (around 200 kHz). The frequency at which 'RF' becomes 'Microwave' is less clear, but is usually considered to be in the region of a few GHz. It is common in the calibration community to make the distinction between the 'DC and low frequency AC and 'RF and microwave' fields at around 1 MHz. This chapter will consider the measurement of RF voltage in...