R&S FSH3 Measurements
The block diagram below shows how a spectrum analyzer works.
Local oscillator
IF filter
Envelope
detector
Video
filter
Detector
Mixer
Input precis.
attenuator
X deflection
Y deflection
RF input
Sawtooth
generator
(for sweep)
Screen
The precision attenuator at the input of the spectrum analyzer adjusts the level of the measurement
signal to the level range that the mixer can handle without overdriving it. The precision attenuator at the
input of the R&S FSH3 is adjustable in 10 dB steps from 0 dB to 30 dB and is directly coupled to the
reference level setting.
The mixer converts the RF input signal to a fixed IF. Conversion is usually performed in several stages
to an IF for which good narrowband IF filters are available. The R&S FSH3 has three mixing stages with
the IFs 4031 MHz, 831.25 MHz and 31.25 MHz.
A local oscillator that can be tuned from 4031 MHz to 7031 MHz is used for conversion to the first IF so
that a certain input frequency is converted to the first IF. The further conversions are performed by
single-frequency oscillators.
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.
1145.5973.12 4.91 E-6
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