Datasheet

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APPLICATION NOTEInterference Hunting
In looking at interference, it helps to understand how receivers
filter out unwanted signals. As shown in Figure 1, the received
or assigned channel is in blue. Interference can occur when
there is energy inside the preselector of the victim receiver.
This means if there is any energy falling under the pre-
selector passband it could impact the performance of the
receiver, by blocking the receiver directly or through a form of
desensitization.
Interference can be particularly prevalent in metropolitan areas
where tower space is limited. The solution to the limited real
estate problem is to multi-couple radio systems through a
system of filters, combiners and isolators. At least, that’s the
theory. In practice it is a complex task to correctly design and
install any multi-coupled radio system. With multiple high-
power transmitters in these systems it is not uncommon to
have a system level failure with the resultant intermodulation
products broadcast through the common antenna.
Identifying interference
Many radio systems have built in spectrum analysis tools.
These tools are useful but they are limited in their ability to
characterize signals because they analyze the problem from
a protocol perspective. In addition, often times they offer
limited context as they view the spectrum of a signal channel.
A spectrum analyzer is a better choice as it’s capable of
measuring a broader portion of spectrum so that problems in
an adjacent spectrum can be identified.
When on the hunt for interference the first challenge is to
determine if it’s even possible to measure the interfering signal.
The challenge is that radio receivers are able to detect very
small signals. Therefore, the spectrum analyzer must be set up
to closely mimic the sensitivity of the victim receiver to “see”
what the receiver is “seeing.” For example, an average LTE
receiver has a sensitivity in the area of -120 dBm. This means
that any RF pollution on the receiver channel that is greater
than -120 dBm can affect the operation of the receiver.
There are two controls in a spectrum analyzer to adjust
sensitivity: reference level (RefLvl) and resolution bandwidth
(RBW). The challenge is that when taking measurements “over
the air” (OTA), the reference level needs to be kept fairly high
(-30dBm), so that the spectrum analyzer doesn’t become
overloaded with all of the RF energy being measured. In
order to improve sensitivity, the spectrum analyzer must be
configured with a narrow RBW filter.
In most spectrum analyzers the RBW control is set
automatically based on the frequency span that the user has
configured. In OTA measurements RBW values should be
reduced in order to see small signals that could be affecting
the victim receiver. This combination results in a very slow
sweep rate for most battery-powered spectrum analyzers
which means that it’s almost impossible to see intermittent
low-level transient signals.
FIGURE 1. Interference can occur when there is RF energy not directly on
channel.
Frequency
Interference
Rx Channel
Rx Band
Preselector
Power