R&S FSH How a Spectrum Analyzer Works
Functions of the Digital Modulation Analyzer
Operating Manual 1173.6275.12 - 30 356
Figure 11-2: Block diagram showing the basic functionality of a spectrum analyzer
The precision attenuator at the R&S FSH input attenuates the signal to a level that the
mixer can handle without overdriving the mixer. The attenuator is directly coupled to
the reference level. You can attenuate the signal in the range from 0 dB to 40 dB in
steps of 5 dB.
The mixer converts the RF signal to a fixed intermediate frequency (IF). This process
usually involves several stages. It lasts until you get an IF for which good narrowband
filters are available. The R&S FSH needs three mixing stages to get an IF that the filter
can handle. Figure 11-3 graphically shows the mixing process.
For models with a frequency limit of 3.6 GHz, the IFs are 4892.8 MHz, 860.8 MHz and
54.4 MHz. The conversion from a specific input frequency to the first IF is done by a
local oscillator (LO). This LO can be tuned from 4.8 GHz to 8.4 GHz. All other
conversions are handled by single-frequency oscillators.
In case of models with a frequency limit of 8 GHz, the IFs are 8924.8 MHz, 860.8 MHz
and 54.4 MHz. The conversion from the first to the second IF for these models is done
by a second local oscillator.
The frequency of the local oscillator determines the input frequency at which the
spectrum analyzer performs measurements:
f
in
= f
LO
– f
IF
.
The first mixer produces the sum frequency f
LO
+ f
in
(= image frequency f
image
) as well
as the difference frequency f
LO
– f
in
.
The image frequency is rejected by the bandpass at the IF so that it does not interfere
with the subsequent frequency conversions.
Figure 11-3: Mixing process
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