214ConceptsTime Gating ConceptsNoise measurements using Time GatingTime gating can be used to measure many types of signals. Noise and noise-likesignals are often a special case in spectrum analysis. With the history of gatedmeasurements, these signals are especially noteworthy.The average detector is the best detector to use for measuring noise-like signalsbecause it uses all the available noise power all the time in its measurement. Thesample detector is also a good choice because it, too, is free from the peak biases ofthe peak detector, normal and negative peak detectors.When using the average or sample detector, noise density measurements using thenoise marker or band/interval density marker can be made without any considerationof the use of gating--gated measurements work just as well as non-gatedmeasurements. Thus, the average detector is recommended for noise densitymeasurements.Older analyzers only had the gated video version of gating available, and these onlyworked with the peak detector, so the rest of this section will discuss the trade-offsassociated with trying to replicate these measurements with an X-Series analyzer.Unlike older analyzers, X-Series analyzers can make competent measurements ofnoise density using the noise marker with all detectors, not just those that are idealfor noise measurements. Thus, X-Series analyzers can make noise densitymeasurements with peak detection, compensating for the extent to which peakdetection increases the average response of the analyzer to noise. When comparing agated video measurement using the noise marker between an X-Series and an olderanalyzer where both use the peak detector, the X-Series answer will beapproximately correct, while the older analyzer will need a correction factor. Thatcorrection factor is discussed in Keysight Technologies Application Note 1303,Spectrum Analyzer Measurements and Noise, in the section on Peak-detected Noiseand TDMA ACP Measurements.When making measurements of Band/Interval Power or Band/Interval Density, theanalyzer does not make compensations for peak detection. For best measurementswith these marker functions, average or sample detection should be used.
