Receiver Dynamic Range: Part 1

The task of the radio receiver has always been to “get the signal.” However, with the proliferation of high-powered transmitters and the burgeoning growth of electronic noise pollution, often weak-signal reception is difficult, if not impossible. Receiver dynamic range is the measure of a receiver’s ability to handle a range of signal strengths, from the weakest to the strongest. Because of the severe dynamic range requirements placed on modern receivers, it is imperative to define rational criteria for evaluating receiver performance. This two-part article provides a tutorial review of receiver dynamic-range specifications and measurements. It discusses the limits and applicability of the various measurements, highlighting potential errors and misleading specifications. Procedures for estimating and measuring true receiver performance are recommended. PRIMARY MEASUREMENTS Primary measurements that affect receiver dynamic range include: noise figure, secondorder intercept, third-order intercept, 1-dB compression, phase noise, internal spurs and bandwidth. This group of receiver measurements is considered primary because most other receiver dynamic-range measurements can be predicted from them. Noise Figure The most common expression of noise figure is the ratio (in dB) of the effective receiver input noise power with respect to -174 dBm/Hz. This single number dominates those receiver characteristics which are generally described as sensitivity. It also describes the “noise floor” of most dynamic-range measurements. To determine noise figure accurately, it should be measured at a pre-detected output of the receiver: that is, at any output which is a version of the received input modified only by linear amplification, frequency translation, and bandwidth. Because noise figure degrades with each successive stage of the receiver, the most desirable measurement port is the audio output. Measurement at this port can be accomplished in either the cw or ssb mode because both of these are pre-detection modes. Note, however, that some receivers may not use a true product detector for cw detection, and the apparent noise figure will be degraded. In the case of receivers that do not have pre-detected audio outputs, the IF output may be used for noise-figure measurements. The most rigorous measurement will require the selection of the narrowest available IF bandwidth because it is under this condition that the largest number of receiver stages are in the signal path.