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R&S FSH
Measurements
1145.5973.12 4.161
E-14
Frequency
Local
oscillator
f
LO
Intermediate
frequency
f
IF
Input
frequency
f
in
Image
frequency
f
image
Input frequency range
0 Hz
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 kHz to 1 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.
Time
Amplitude
IF signal (AM)
Envelope signal
Time
Amplitude
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