R&S FSH
How a Spectrum Analyzer Works
Deleting Datasets
Operating Manual 1309.6275.12 - 14
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Figure 9-3: Mixing process
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.
