The wireless industry has been focused on Passive Inter-‐Modulation (PIM) performance since the introduction of LTE. A system with high PIM levels suffers a degraded RF channel quality in the form of higher noise levels. This reduces the signal to interference & noise ration (SINR) and will nullify much of the data rate benefits that LTE offers. This article deals with return loss (RL) as it affects PIM measurements for in-‐building distributed antenna systems (DAS).
PIM testing consists of two narrowband signals at frequencies f1 and f2 being transmitted into a DAS. If the test signals encounter a PIM generating source
(typically some form of metal discontinuity), PIM interference products are generated. The PIM tester is designed with a high sensitivity, narrowband receiver tuned for the low side PIM frequencies defined by fpim = 2f1 –f2. The power of the PIM test signals f1 and f2, are proportional to the power of the PIM product fpim. A lower test power results in a lower PIM power and vise versa.
Return Loss (RL), is a measurement of how well a system (or component) transmits RF energy. A high RL (40 dB for example) indicates that virtually all of the RF energy transmits through the system. Conversely, a low RL (10 dB for example) indicates that the some imperfection in the system is reflecting power and not allowing the RF power to fully transmit through the system.
As an example consider the DAS section illustrated below in figure 1:
figure 1. DAS Section Under Test
Assume the Pim tester transmits two test signals, f1 and f2, into the DAS. When the test signals reach the PIM source at Ant2, a PIM interference product fPIM is created and transmits back to the PIM tester. Assume for this example that fPIM is received at the PIM tester as a fail.
Now consider that a RL failure occurs at splitter, Spt1. This could be due to a bad splitter, loose connector or other RL event. The result is that the test signals are unexpectedly reduced in power for all DAS elements beyond Spt1. Thus less test power is being presented to the PIM source at Ant2. This lower test signal power at Ant2 results in a lower PIM power. Additionally, the reduce PIM power is further reduced passing through the RL failure at Spt1. This can result in a falsely lower PIM product, fPIM, being received at the PIM tester and could result in a false pass being recorded.
Poor return loss performance can create false passing PIM values.
As a result of the PIM tester’s receiver being tuned so that only the PIM interference product is received, no other signals are registered. If, for instance, the two test signals, f1 & f2, are reflected back into the tester due to a RL failure, this has no impact on the recorded PIM result. Without completing a full RL characterization of the DAS, PIM results cannot be considered valid.
As best practices GWI Telecom follows a disciplined RL measurement practice that includes testing all cables and antennas as well as “mini” branch RL testing into each splitter section. In figure 1 above, this would include a RL measurement into cable feeding Spt2 and then again into the cable feeding Spt1. This RL branch test verifies that all cable and antenna connectors are properly done as well as verifying RL performance of the splitters. Unless passing RL can be verified for all DAS sections and branches, PIM results cannot be considered valid.
No PIM testing should be conducted prior to RL verification.
The best system has a maximum RL value (excellent RF transmission through the DAS) and minimum PIM (reduced interference sources). If RL weak points are left within a DAS they can mask PIM problems and could ultimately result in incorrectly identified PIM problems as passing. Both RL and PIM results are essential to proper DAS testing.