Datasheet

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APPLICATION NOTEInterference Hunting
When analyzing interference problems, the RBW control is
very important. The RBW filter helps us discriminate signals
in the spectrum display so that we can determine individual
sources of RF energy within the span we are monitoring. Figure
2 illustrates why the RBW setting is so important. If RBW is too
wide, as it is on the left side, the spectrum analyzer is going
to miss smaller signals that might be close to a larger signal.
With a very narrow RBW, the spectrum analyzer can easily
discriminate between the two signals that are close together.
However, a narrow RBW will slow down the spectrum analyzer,
meaning that a longer signal duration is required in order to
guarantee probability of intercept.
Breaking down spectrum analyzers
In broad terms, there are two types of spectrum analyzers on
the market today: swept-tune spectrum analyzers and real-
time spectrum analyzers. Since getting out into the field is
required to locate interference sources, swept-tune spectrum
analyzers – a technology developed some 40 years ago – have
long been the tool of choice. The primary reason is simple:
they have been the only low-cost, portable, battery powered
spectrum analyzer technology available. As noted previously,
this situation has now changed with the introduction of USB-
based real-time spectrum analyzers from Tektronix.
With traditional spectrum analyzers, the instrument sweeps in
frequency from left to right. If you’re looking for a signal that’s
not there very often, the fact that the instrument is sweeping
is a problem since the offending signal could be on and off
in between sweeps and never captured. The problem is
personified by the probability of intercept or POI specification.
The principle of probability of intercept is defined as “what is
the minimum duration of the interfering signal to guarantee that
the spectrum analyzer will be able to visualize the signal?”
As you might imagine, given their sweeping architecture,
swept-tune spectrum analyzers have a low probability of
intercept, meaning that the interfering signal may have to
be on for a long period of time for a sweeping analyzer to
fully characterize the signal. RTSAs are able to continuously
measure the spectrum for any span up to their maximum real-
time span. This means that the POI is very good even for very
short duration signals. What’s more, operators are able to get
the information much faster and even record and store live
spectrum data for later analysis.
FIGURE 2. The RBW filter improves the sensitivity of spectrum measurements. In
general, with a narrower RBW filter the spectrum analyzer will be more sensitive,
making it possible to measure small signals.
level
freq
level
freq
RBW
Speed
Wide Narrow
level
freq
Faster Slower