Specifications

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