R&S FSH
How a Spectrum Analyzer Works
Deleting Datasets
Operating Manual 1309.6275.12 - 14
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The video filter comes after the envelope detector. The filter is a lowpass with an
adjustable cutoff frequency which limits the bandwidth of the video signal. It is
particularly useful when sine signals are to be measured in the vicinity of the spectrum
analyzer’s intrinsic noise. The sine signal produces a video signal that is a DC voltage.
At the IF, however, the noise is distributed over the whole bandwidth or, in the case of
the video signal, over half the bandwidth of the resolution filter. By selecting a narrow
video bandwidth relative to the resolution bandwidth, the noise can be suppressed,
while the sine signal to be measured (= DC) is not affected.
The figures below show a weak sine signal. In the first picture, it is measured with a
large video bandwidth and in the second with a narrow video bandwidth.
Limiting the video bandwidth smoothes the trace considerably. This makes it much
easier to determine the level of the measured signal.
The detector comes after the video filter. The detector combines the measured
spectrum so that it can be represented as one pixel in the trace. The R&S FSH uses
631 pixels to form the trace, i.e. the whole measured spectrum has to be represented
using just 631 pixels. Common types of spectrum analyzer detectors are the peak
detector (PEAK), the sample detector (SAMPLE) and the RMS detector (RMS). An
Auto Peak detector which simultaneously displays the maximum peak and the
minimum peak is usually also provided. The Fig. below explains how these detectors
work.
