PIM is only generated when there are 2 or more signals present on a transmission line, so we need 2 signal generators. The power levels present in a typical base station are much higher than those that are at the output of a signal generator, so we must amplify these signals to the proper levels. High power (20 watts or more) amplifiers are very expensive and at Amphenol RF we have 4 amplifiers. Two for the 900 Mhz range and two for the 1700-1900 range.

In order to combine the two amplified signals, we feed them into a device called a diplexer which is basically a signal combiner. The single, combined output of the diplexer is then fed into a duplexer which is a device that is similar to a power divider with very narrow band filters. The cable assembly or connector under test is then attached to the output of the duplexer and terminated with a low PIM load. At Amphenol RF we manufactured or own termination consisting of a very low PIM Amphenol RF connector attached to a 100 meter long reel of .141 conformable cable. This provides a load that can handle the power level required and generates PIM at levels below -135 dBm.

At this point, there are 2 alternative ways of measuring the actual PIM signal. Reflected and transmitted. The PIM that is generated in the Device Under Test (DUT) is propagated in both the forward and reverse direction. The advantage of the transmitted method is that the measured value will be relatively flat over the frequency range so sweeping is not necessary as compared to the reflected method which will phase in and out if you are measuring a relatively long cable assembly. This necessitates sweeping in order to confirm you are seeing the maximum PIM in the test frequency range. The disadvantage of the transmitted method is that it requires a second duplexer (more expensive) and is prone to reflecting harmonics of the incident signal since it is not matched at the higher frequencies at which the harmonics are generated. The PIM signal is then fed through a bandpass filter into a low noise amplifier (LNA) and measured on a spectrum analyzer.