In RF work, as in low-frequency work, test instruments generally are designed either to analyze signals, or to determine parameters of components and systems. Signal analysis ranges from simple voltage, current, and power measurements to measurements of frequency, relative phase, frequency stability, modulation characteristics, RF field strength, and spectrum analysis. Parameter measurements range from simple resistance and reactance measurements through measurements of transfer characteristics of linear multiport devices (filter shapes, amplifier frequency response, etc.), and distortion (dynamic range, intermodulation). Parameter measurements require a signal generator to provide the stimulus whose response is measured. It is now common for measurement instruments to include built-in signal generators, but free-standing signal generators are widely used in specialized test setups.

Power measurements

Power measurements, as pointed out in Chapter 18, are best made with a square-law detector. If the waveform is unknown or noisy (i.e., has a random component), a square-law detector must be used, as power, by definition, is proportional to the average square of voltage. If the signal waveform is known, e.g., a sine wave, the power can also be calculated, for example, from a measurement of 〈|V(t)|〉. Commercial RF power meters normally present a 50-ohm impedance to source being measured, allowing a reflectionless connection to a 50-ohm cable.

Thermistor (temperature-dependent resistor) power meters are true square-law instruments and have a dynamic range typically from 1 μW (−30 dBm) to 100 mW (20 dBm). The power to be measured heats a thermistor which is one leg of a Wheatstone bridge circuit.