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TEST AND MEASUREMENT
broadband noise covering the entire uplink band, rather than
single interfering frequencies.
The goal of field engineers and technicians is to reduce
the PIM magnitude so it doesn’t negatively impact site
performance. This is achieved by finding the PIM sources in
the RF path and making them more linear. Improving system
linearity reduces the magnitude of all IM products, with the
objective being to drive those that fall in the Rx band below
the receiver noise floor.
PIM. There are recorded instances in which simply tightening
the connections with a torque wrench improved the PIM
performance of a distributed antenna system (DAS) by 20 dB.
External PIM Sources
PIM sources are not only inside the coaxial cables, PIM can
be caused by loose metal-to-metal junctions in the RF path
beyond the antenna. Loose or rusty mounting hardware,
metal structures near the antenna, or metal objects in front of
the antenna all can be sources of PIM.
The image on the previous page provides
insight into what PIM looks like at a cell site.
This is an example where universal mobile
telecommunications system (UMTS) carriers
at 2100 MHz are mixing with Groupe Spécial
Mobile (GSM) signals at 1900 MHz at a macro
site.
Whenever PIM rises above the noise floor,
it impacts site performance. In this case, the
PIM magnitude is 30 dB above the noise floor,
resulting in reduced cell size and increased
dropped call rates.
PIM can have another negative impact on the
customer experience – the high noise levels
it causes requires greater Tx power by the
mobile device, which results in shorter battery
life for the consumer’s UE. At minimum, this
can lower throughput for that customer –
costing operators potential revenue and, at
maximum, it will increase customer churn.
PIM Frequencies
Low-order IM products, such as IM3 and IM5, are more likely
to cause interference than high-order IM products, such as
IM7 and IM9. Different PIM sources behave differently but
in general, IM3 is the highest magnitude product. IM5 is
typically 20 dB lower than IM3, IM7 is typically 15 dB lower
than IM5, and so on. Depending on the system linearity, IM
products below IM5 may already be low enough that they
don’t cause interference. This doesn’t mean that high-order
IM products can be ignored, only that the likelihood they will
cause interference is lower than the chances of IM3 and IM5
creating interference.
PIM bandwidth increases as the carrier bandwidth and IM
order increase (i.e. IM3 = 3x the carrier bandwidth, and IM5
= 5x the carrier bandwidth). As a result, PIM often appears as
The test Process
The best way to measure linearity is with a PIM analyser – a
specialised test instrument used to excite non-linearities in
the RF path. The instrument sends two continuous wave (CW)
test signals into the system under test, and then monitors
the magnitude of the noise generated by these test signals.
PIM testers typically measure the 3rd order intermodulation
product as a way to characterise the linearity of the system.
If IM3 is low, the linearity is good. Conversely, if IM3 is high,
repairs are needed.
While PIM is a growing concern, it's not the only issue
field technicians have to worry about. They must also
conduct sweeps to ensure the performance of antenna line
systems. For years, field technicians had to use two separate
instruments for these equally important measurements, as
they are distinctly different. A sweep tester was necessary to
measure how efficiently signals passed through a system.
Then, a PIM tester was needed to measure the noise
generated when those signals passed through the system.
A sweep tester – also known as a cable and antenna
analyser – transmits a single test frequency at low power and
measures how much energy bounces back at that frequency.
The ratio of power received vs. power transmitted is displayed
as a data point on the screen. The instrument then steps to a
new frequency and repeats the process until a full start – stop
frequency range has been measured.
Because a cable and antenna analyser only transmits
a single test frequency, it’s not capable of generating
PIM. Defects that cause PIM may or may not cause high
reflections. Likewise, defects that cause reflections may or
may not cause PIM. The presence of one is not an indication
of the other. Therefore, both tests must be performed to
assure the quality of the RF infrastructure.
Dual analysers
There are instruments that can fully certify both PIM and
sweep performance, allowing field technicians to carry fewer
instruments to a site, and to hoist just one analyser to the top
of the tower for remote radio head testing.
Another benefit to combining PIM and antenna
sweeping into a single instrument is that field technicians
have the ability to overlay the active DTP measurement
with a distance-to-fault (DTF) measurement. When a DTP
measurement is made, the instrument automatically conducts
a DTF first and displays the overlay. DTF traces act like maps
to very clearly show RF connector locations. Since PIM often
occurs at RF connections, field technicians can use these highresolution DTF plots to help identify bad connections.
Field technicians responsible for maintaining the quality
of wireless networks must efficiently locate PIM sources
to quickly and effectively eliminate interference that can
lower network performance. Utilising a test solution that can
effectively measure PIM and conduct antenna analysis will
ultimately reduce test time and lower costs. n
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