R&S FSH
How a Spectrum Analyzer Works
Functions of the Digital Modulation Analyzer
Operating Manual 1173.6275.12 - 30
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The first local oscillator is tuned with a sawtooth which simultaneously acts as the x
deflection voltage for the display. In practice, synthesizer technology is used to
generate the frequency of the first local oscillator and for a digital display.
The instantaneous sawtooth voltage therefore determines the input frequency of the
spectrum analyzer.
The bandwidth of the IF filter at the IF determines the bandwidth that is used for
measurements. Pure sine signals are passed by the IF filter characteristics. This
means that signals closer together than the bandwidth of the IF filter cannot be
resolved. This is why the bandwidth of the IF filter in a spectrum analyzer is referred to
as the resolution bandwidth. The R&S FSH has resolution bandwidths from 1 Hz to
3 MHz.
The bandlimited IF is passed to the envelope detector. The envelope detector removes
the IF from the signal and outputs its envelope. The output signal from the envelope
detector is referred to as the video signal. As it has been demodulated, it only contains
amplitude information. The phase information is lost.
With RF sine signals, the video signal is a DC voltage. With AM signals the video
signal contains a DC component whose amplitude corresponds to the carrier power
and an AC component whose frequency is equal to the modulation frequency, provided
the modulation frequency is inside the resolution bandwidth.
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.
