Boonton PIM 21 User Manual - Manual (Page 9)

9 Boonton
The PIM 21 is a low power tester with excitation carrier powers up to +30/+33* dBm each. Its
minimum size, reliability, and battery operation, all provide true portability.
To test individual components, bench PIM test systems typically use a power of up to 20W per
carrier. Such systems are heavy and bulky, and are generally considered impractical for field
testing, especially at the mast head.
Measurements between 20W bench sets and 2W portable field sets show predictably some
what different readings. This does not at all diminish the value of the low power measurement.
In theory, a 3rd order IM product will increase by 3 dB for every 1 dB increase in carrier power,
however in practice increases are very much dependent on test conditions and are considerably
lower than the theoretical 3 dB per 1 dB change in power. This is believed to be due to the fact
that poor or corroded connections act much like a diode, and become abruptly non-linear when
sufficient current excites them. This current is usually very low so the PIM 21’s output power is
usually adequate.
The following chart shows 1W and 2W measurement results compared with the result seen
on a 20W bench tester for the same component. There is only a 10 dB difference between PIM
measured at 2W and 20W. The arrows on the chart that follows indicate that a -153 dBc 2
Watt measurement is the equivalent to a -143 dBc at 20W.
Many tests have been performed and results show that a good rule of thumb is about 10 dB. It
should be noted that in certain circumstances the difference could be as much as 20 dB.
Users in the field have found that system components such as antennas and feeder cables
generally do fail PIM with an abrupt change in PIM performance and the 10 dB rule of thumb
holds. A measured PIM below -140 dBc (Green LED’s) is a passing component, while a compo-
nent measuring PIM above -140 dBc (Red LED’s) indicated a problem. This allows identification
of problem components with the PIM 21, in spite of the lesser sensitivity.
*Model Dependent