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APPLICATION NOTEInterference Hunting
Understanding interference
Fundamentally, interference is associated with degraded
performance. It usually means that a signal is impacting your
system or device causing a coverage issue, a reception issue,
or an access issue. To put it simply, interference is associated
with unwanted energy at the wrong place and at the wrong
time. Here are the main types of interference you’re likely to
encounter in one form or another:
• Co-channel interference is when more than one transmitter
is on the same channel. In today’s crowded spectrum,
pretty much every frequency is being shared by some other
person or service.
• Adjacent channel interference comes from the energy from
another transmitter. It could be nearby geographically or
close to your operating frequency with much higher power.
• Impulse noise results from imperfect shielding allowing
energy to leak out and interfere with RF devices. Noise can
also result from a partial failure in a device or from other
sources such as industrial machinery.
Type and Source of Intermodulation
Interference
For anyone in spectrum management, sooner or later you
will need to track down an intermodulation product that is
causing interference. They can often occur intermittently
and because there are multiple transmitters involved, they
can be hard to identify. They will usually fall into one of the
following three categories.
Transmitter generated intermod — The transmitted signal
from one or more transmitters is received at the output
of another transmitter. This received signal must be of
sufficient strength to mix with the transmitter’s own carrier
in a non-linear final amplifier. This newly mixed signal is
then amplified and transmitted along with the desired
carrier.
Receiver generated intermod — Transmitted RF
energy can sometimes generate a voltage in a nearby
receiver’s RF amplifier resulting into a non-linear state
that allows it to act as a mixer that in turn become an
unwanted intermod product. This undesired signal can
be received via the antenna input or can sometimes be
coupled directly into the receiver. Receivers can also be
desensitized by intermod.
Externally generated intermod — Loose mechanical
connections or corroded metal connections form non-
linear electrical junctions and can act as unintended
"diodes" or mixers. When these devices are excited by
sufficient levels of one or more signals they generate
intermod products. Major offenders are tower section
joints, broken welding beads, improperly seated or
corroded connectors in the RF chain, metal buildings, and
chain-link or barbed-wire fences.