High-speed digital data communications used in today’s wireless
networks have made passive intermodulation (PIM) testing
critical. As cell usage and throughput grows, the peak power
produced by new digital modulations increases dramatically,
contributing heavily to PIM problems.
PIM lowers the reliability, capacity and data rate of cellular
systems by limiting the receive sensitivity. Indications of PIM
include receive-noise-floor-diversity-imbalance and high noise
floors. Other signs include shorter average call duration, higher
dropped call rates, lower data rates, and lower call volume.
Two or more strong signals and a nonlinear junction create
passive intermodulation. Strong signals normally come from
transmitters sharing an antenna run, transmitters using adjacent
antennas, or nearby towers with conflicting antenna patterns.
Damaged or poorly torqued RF connections, contamination,
fatigue breaks, cold solder joints, and corrosion can create
nonlinear junctions. In addition, nearby corroded objects, such as
fences, barn roofs or rusty bolts, can cause PIM. This is referred
to as “Rusty Bolt Effect.”
PIM often appears as poor statistics from the affected sector.
One of the first and most direct indications can be seen in cells
with two receive paths. If the noise floor is not equal between
the two paths, the cause is likely PIM generated inside the noisy
receive path, a condition called Receive Diversity Noise Floor
Reflective PIM Testing
Reverse, or reflective, testing is the most commonly used PIM
test. Two signals are sent to an antenna and the same test port is
used to capture and measure passive intermodulation. Because
reflective PIM testing is affected by the electrical length of the
antenna cable, results may not be accurate at fixed frequencies.
Reverse PIM tests should be done while sweeping one of the
frequencies to avoid unintentional partial error signal cancelation.
Another option is to change one of the test frequencies to see how
the result varies.
Forward PIM Testing
A forward PIM test can be performed in two ways – using
filter networks on the output of a device under test (DUT) or an
external antenna measuring propagated signals, such as an antenna
in an anechoic chamber. This test can be very handy when trying
to find external materials causing PIM, such as loose flashing or
other metalwork used in building construction.
PIM testing is becoming more critical as cellular systems age
and the carrier count increases. A cell site constructed with PIM
in mind will cost less to maintain over time, and show cleaner
performance than similar sites that were not PIM tested.