Boonton PIM 31 User Manual - Manual (Page 78)

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11.1.3 Environment
• Daily temperature variations, thermal loading by the sun and RF heating
vary junctions and can cause, often intermittent, PIM distortions.
• Wind-induced vibrations vary junctions, and can weaken or break down
joints.
• Airborne dirt and moisture cause oxidation of materials and cause PIM
distortion.
The antenna in the picture shows
oxidation within the power divider.
Tests with vector analyzer line
sweep test would not reveal the
problem; however PIM testing
detected the issue immediately
11.2 How to test PIM
PIM testing for field applications requires the injection of two CW signals (f1 and
f2) into a system under test. Intermodulation products (IM) of the 3rd, 5th, 7th…
order, caused by faulty components, appear immediately. The strongest
intermodulation product is usually that of the 3rd order (IM3). Frequencies of
these intermodulation products can be calculated as follows:
f
IM3L
= (2 x f1) – f2
f
IM3H
= (2 x f2) – f1
The picture below shows an example of passive intermodulation. Frequencies f1
(869 MHz) and f2 (894 MHz) are located in the Tx range, causing intermodulation
fIM3L (844 MHz) and fIM3H (919 MHz). Both IM products can cause serious
interference.