November 29, 2016
It is approaching 20 years since the original IEC62037 recommendation to use 20W test power for PIM tests was first published. Given the many changes that have occurred in mobile communications technology in the past two decades, the question of test power is once again being actively discussed within the IEC working group. It will be interesting to see where that discussion leads!
One argument sited for not changing the “standard” PIM test power is that PIM sources behave in a very predictable manor vs. power. If you know the IM3 level of a system at one test power level you can predict the PIM level at a different test power level using the “PIM slope” for that system. Theory states that IM3 will change 3 dB for every 1 dB change in test power. In practice, the PIM slope typically falls somewhere between 2.2 to 2.8 dB/dB. Many operators use a value of 2.5 dB/dB when extrapolating test results, accepting the fact that actual results could be off by a few dB.
But what about cases where the PIM slope is not consistent vs. power? In those instances, test results from one power level will not accurately predict PIM results at a different power level. Such a case was recently observed in a system containing a combination of internal and external PIM sources. When the PIM of this system was measured over a range of power levels, two distinctly different PIM slopes were revealed, as shown in figure 1.
A Case for 40W
Distance-to-PIM measurements were made on this system using the Anritsu PIM Master™ MW82119B. The results helped to reveal the reason for the strange PIM vs. Power results. At low power levels, a PIM source inside the feed system is dominant. This internal PIM source establishes the PIM slope for power levels below 25W (44 dBm). At power levels above 25W a second PIM source beyond the antenna becomes dominant. This second PIM source has a “steeper” PIM slope resulting in higher PIM values as the power level increases.
In this case, PIM testing the system using 20W test tones does not give an accurate indication of how the system performs at higher power levels, evident by the three screen shots in figure 2. Testing with 40W test tones not only provides a more accurate indication of actual noise levels, but also exposes the dominant PIM problem in the system. If your standard 20W test results show the system to “pass,” but system noise levels are still poor, try increasing your PIM test power. You may find that the PIM level is increasing faster vs. power than expected, as was the case here.
As you can see from this example, identifying PIM is no easy task for field technicians and engineers. To help locate external sources, you can download a new white paper entitled Identifying Sources of External PIM.