Radio frequency measurements - Measurement methods and equipment

In this article, we explain the principles, methods and means of measurement of the four main quantities used in the radio frequency and microwave field (between 100 kHz and 100 GHz). These quantities are voltage, power, noise and impedance. The aim of this article is to describe the different measuring devices used in the radio frequency field, to present their field of application, to give their performance and measurement uncertainties, and to discuss the measurement techniques associated with these different devices.

In the first section on voltage measurement, we present the different types of voltmeter, such as the broadband AC voltmeter, the sampling voltmeter and the RF millivoltmeter. The oscilloscope as a voltage meter is also presented. Voltage measurement at radio frequencies (frequencies above 100 kHz) poses a number of special problems: firstly, the amplitude of the voltage varies periodically along a mismatched line, and voltage measurement only makes sense if the plane in which it is measured is specified; secondly, the notion of power takes precedence over that of voltage as frequency increases.

In the second section on power measurement, we look in detail at the three most commonly encountered types of detectors: bolometer detectors, thermocouple detectors and diode detectors, all of which are generally used for low-power applications (up to around 1 W). Secondly, we present other types of power meter, particularly for high-power applications. For frequencies below a few hundred kilohertz, power is commonly calculated from voltage measurements, provided that the load impedance is known precisely (load impedance of 50 Ω or 75 Ω, for example). Conversely, when the frequency is higher than a few tens of megahertz, it's easier to measure power directly and deduce voltage and current by calculation.

In the third section on noise measurement, we present the different types of instrument for measuring noise equivalent temperature and noise figure. We present the main noise measurement methods, such as the radiometer equilibrium method for noise temperature, and the Y-factor method for noise figure.

In the final section, we discuss impedance measurements. The three main measurement methods are presented: impedance determination by self-balancing bridge, the current/voltage method and the use of a network analyzer, determining impedance from the reflection factor. Other instruments less widely used today, such as the various impedance measurement bridges and the Q-meter, which uses a resonance method, are also described.