Agilent Spectrum Analysis Basics - Agilent Technologies
Agilent Spectrum Analysis Basics - Agilent Technologies
Agilent Spectrum Analysis Basics - Agilent Technologies
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EMI detectors: average and quasi-peak detection<br />
An important application of average detection is for characterizing devices<br />
for electromagnetic interference (EMI). In this case, voltage averaging, as<br />
described in the previous section, is used for measurement of narrowband<br />
signals that might be masked by the presence of broadband impulsive noise.<br />
The average detection used in EMI instruments takes an envelope-detected<br />
signal and passes it through a low-pass filter with a bandwidth much less than<br />
the RBW. The filter integrates (averages) the higher frequency components<br />
such as noise. To perform this type of detection in an older spectrum analyzer<br />
that doesn’t have a built-in voltage averaging detector function, set the<br />
analyzer in linear mode and select a video filter with a cut-off frequency<br />
below the lowest PRF of the measured signal.<br />
Quasi-peak detectors (QPD) are also used in EMI testing. QPD is a weighted<br />
form of peak detection. The measured value of the QPD drops as the repetition<br />
rate of the measured signal decreases. Thus, an impulsive signal with a given<br />
peak amplitude and a 10 Hz pulse repetition rate will have a lower quasi-peak<br />
value than a signal with the same peak amplitude but having a 1 kHz repetition<br />
rate. This signal weighting is accomplished by circuitry with specific charge,<br />
discharge, and display time constants defined by CISPR 13 .<br />
QPD is a way of measuring and quantifying the “annoyance factor” of a signal.<br />
Imagine listening to a radio station suffering from interference. If you hear<br />
an occasional “pop” caused by noise once every few seconds, you can still<br />
listen to the program without too much trouble. However, if that same<br />
amplitude pop occurs 60 times per second, it becomes extremely annoying,<br />
making the radio program intolerable to listen to.<br />
Averaging processes<br />
There are several processes in a spectrum analyzer that smooth the variations<br />
in the envelope-detected amplitude. The first method, average detection, was<br />
discussed previously. Two other methods, video filtering and trace<br />
averaging, are discussed next. 14<br />
13. CISPR, the International Special Committee on<br />
Radio Interference, was established in 1934 by a<br />
group of international organizations to address<br />
radio interference. CISPR is a non-governmental<br />
group composed of National Committees of the<br />
International Electrotechnical Commission (IEC),<br />
as well as numerous international organizations.<br />
CISPR’s recommended standards generally form<br />
the basis for statutory EMC requirements adopted<br />
by governmental regulatory agencies around the<br />
world.<br />
14. A fourth method, called a noise marker, is<br />
discussed in Chapter 5, “Sensitivity and Noise”.<br />
A more detailed discussion can be found in<br />
Application Note 1303, <strong>Spectrum</strong> Analyzer<br />
Measurements and Noise, literature number<br />
5966-4008E.<br />
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