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of[email protected] | www.tekbox.net 7TBHDR1 MANUAL
3. APPLICATION AS PRE-AMPLIFIER FOR SPECTRUM ANALYZERS
In applications, where signals close to the base noise floor of the
spectrum analyzer need to be investigated, it may be necessary to turn
on the internal pre-amplifier. However, as pre-amplifiers of spectrum
analyzers are typically optimized for low noise, non-linearities may
create distortions, which falsify the displayed spectrum. Third order non-
linearities are especially critical, as they often appear in the frequency
range of interest.
EXAMPLE:
The picture below shows two signals with an amplitude of -35dBm and frequencies 200MHz, 200.1MHz at the input of a spectrum analyzer
If there is no requirement to look at signals with amplitudes close to
the noise floor, everything is fine. However, if the spectrum has
to be checked for signals with amplitudes close to the noise floor of
approximately -100 dBm, the internal pre-amplifier of the spectrum
analyzer needs to be turned on.
The picture below shows the spectrum of exactly the same set up,
with the pre-amplifier of the spectrum analyzer turned on. It shows two
additional spectral lines which are caused by third order intermodulation
of the internal preamplifier. Assuming there are more than two signals
present at the analyzer, e.g. when doing radiated or conducted noise
EMC pre-compliance testing or when investigating broadcast spectrum.
It will become hard to tell which signals are real and which signals
are intermodulation products generated by the pre-amplifier of the
spectrum analyzer:
