Electronic circuits fall into two broad categories—those that process and transform signals and those that measure signals. Their functions are often combined, as in the IF section of a receiver— which processes the signal (by amplification and demodulation), and also delivers an indication of received signal-strength (the RSSI function), a slowly-varying voltage that may be displayed and/or used for automatic control of variables such as gain and frequency (AGC and AFC).
Circuits that measure RF signal strength, whose fundamental metric is power, are generally called detectors, but only a thermopile (bolometer) measures this quantity directly. Integrated-circuit detectors invariably operate on a voltage sample of the signal to be measured. Circuits of this class are classified by the type of signal transformation they provide.
In 1976, Analog Devices supplied the first monolithic “true-rms” detectors for use at moderate frequencies. Now this product line includes devices, such as the AD8361, that have extended this capability to the microwave domain. The accurate determination of signal power, independent of its waveshape (stated otherwise, its probability density function) is important in modern communications systems such as CDMA. Unlike thermal detectors, these true-rms detectors use analog computation to directly implement the relevant equations—at gigahertz frequencies.