February 23, 2017
As many field technicians and engineers responsible for wireless networks know, the industry-recognized specification for passive intermodulation (PIM) is IEC 62037. To achieve measurement consistency industry wide across networks, this specification recommends using 2x20 Watt (43 dBm) test tones, which was fine during the development and deployment of 2G and 3G systems. The more stringent requirements of 4G and 5G systems, however, has initiated discussions about the need to reassess current test power level recommendations and determine if 40 Watt (46 dBm) test power would better serve current and future high-speed mobile networks.
Better PIM Test Accuracy
The suggested increase in recommended test power levels is necessitated by the need for lower PIM, which means PIM test accuracy will be more important than ever. Not surprisingly, the discussions have raised some questions. If an equivalent test level can be determined for any power level, why does the IEC specification recommend 20W? Why not test at a lower power to make the test safer and reduce the size, thus cost, of the test equipment?
In addition to recommending 20W test power, IEC 62037 requires PIM of the test instrument to be 10 dB lower than the PIM level being measured. Figure 1 shows possible measurement error as a function of the difference between PIM signal level and residual PIM level of the instrument. With an IEC-specified 10 dB difference, a +2 dB/-3 dB measurement error is possible. A 5-dB difference increases the measurement error to +4 dB/-7 dB. This shows a reduction of measurement error due to a signal level increase above the instrument’s residual PIM. At a 20-dB difference, the measurement error would be less than 1 dB.
Figure 1: Measurement error curve from IEC 62037
For every 1 dB increase in test power, the IM3 generated by a device increases by approximately 3 dB. This provides a practical way to boost the PIM signal being measured and potentially increase measurement accuracy.
The table below shows an example of the differences between measured PIM and residual PIM for three items tested in Figure 1. In this example, the PIM analyzer meets the requirement of 10 dB above residual PIM for all devices at all test powers, except for the coupler at 5W. Accurately measuring this low of a PIM device would require an increase in test power to 40 dBm (10W).
As test power increased, test accuracy of all tested devices improved. This is due to the low residual PIM of the test analyzer and the lack of significant increase in residual PIM as the power was raised. This may not be the case for all test instruments but it is a factor that must be considered when considering changes in test power.
Another benefit of increasing test power is the potential for more stringent testing of device linearity. Figure 2 is an excerpt from a manufacturer’s datasheet showing two PIM specifications for a 4.3-10 series RF connector when tested at different test power levels.
In this example, the PIM level specified at 20W testing is -166 dBc (-123 dBm). To accurately verify this specification per IEC 62037, the test analyzer’s residual PIM must be better than -133 dBm. If this residual PIM could also be maintained at 40W test power, the same -123 dBm PIM level could be applied. This level at 46 dBm is -169 dBc, which is a 9 dB more stringent test than currently specified. One aspect to remember is that a residual PIM level of -133 dBm at 40W test power represents the current state-of-the-art in PIM test equipment technology. Levels like this are possible today in lab environments, but they may not be practical to achieve in the field.
PIM Issue Missed by 20W
In the examples shown, the change in PIM vs. test power, or PIM slope, has been constant over a wide range of test powers. In cases where the PIM slope is irregular, the linearity of a device varies as the power changes, and the test power choice can have a significant impact on the result.
Figure 3 shows measurements from a cell site using the Anritsu PIM Master™ MW82119B battery-operated PIM analyzer. It displays a combination of internal and external PIM sources. As you can see, a rapid increase in PIM was observed when measured at higher power levels.
The reason for the non-constant PIM vs. power results was revealed by Distance-to-PIM (DTP) measurements. At lower power levels, a PIM source inside the feed system was dominant. This internal PIM source made the PIM slope for power levels below 25W (44 dBm). At power levels above 25W, a second PIM source beyond the antenna became dominant. This second PIM source had a steeper PIM slope, resulting in higher PIM values as the power level increased.
Figure 4: Distance-to-PIM measurements at different test power levels
As proven by the results shown in Figure 4, PIM testing at either port at 20W did not provide a true indication of the PIM level experienced at higher power levels. Testing with 40W not only provided a more accurate indication of actual noise levels at the site, but also exposed the dominant PIM problem outside the system.