19 Sep 2011

Demystifying PIM – Passive Intermodulation Products (Page 2 of 3)

More importantly, a device that meets the desired PIM performance at 2W, may well fail if subjected to higher power levels of 20W or 40W. PIM tests that are performed at low power can mask PIM non-linearities by not bringing them out. Although performing PIM testing at 40W is considered to be a more stringent test then is currently required, it exposes a cell site’s PIM vulnerabilities in a significantly more quantitative manner leaving little room for conjecture as to the integrity of the device(s) under test.

Meeting the 20W PIM specification today at 40W gives operators and contractors more measurement confidence and allows room for growth. The 20W standard was intended to simulate issues for the power of a single carrier or multiple carriers not exceeding an aggregate power of 20W on a given transmission path. However, this does not guarantee performance if the number of carriers or total aggregate power increases, as is typical with network growth. As a result, testing at 40W accommodates these conditions and can eliminate the need for repeated testing and PIM mitigation in the future.

Is it PIM, or Interference?

While taking PIM measurements at a given site, it is sometimes difficult to differentiate PIM energy, generated as a result of internally transmitted carrier signals, from external interference signals permeating from outside the antenna. PIM testing is intended to be performed within a site’s 50--ohm transmission line path, specifically from 50-ohm line path from the radio to the antenna.

When antenna manufacturers test antennas for PIM performance, measurements are taken in an anechoic chamber where the presence of external interference signals is not possible. In the field, external interference signals can often be construed as PIM signals, because they occasionally fall within the up-link received band. The source of these unwanted signals is usually adjacent cell sites, old TV transmitters, or the presence of metallic structures near the site. Interfering signals from adjacent cell sites or TV transmitters can be identified by using a spectrum analyzer and comparing spectrum responses between sectors to identify the direction of the interference.

Varying noise floor levels between the three sectors will help determine the presence of interference energy coming from local metallic structures, with the sector showing highest noise floor as having the highest level of broadband noise interference.

PIM-Pro’s RX interference function was specifically implemented for the measurement of interfering signals within the RX band. RX interference measurements can quickly pinpoint the direction of interfering signals and assess the relative signal energy strength at each sector. Field technicians can store results and compare all three sector signal strengths.

BTS Receiver Sensitivity Input Power and Measurement Requirements

Figure 2. A PIM-Pro RX Interference screen shot.

PIM non-linearity discussion

PIM non-linearity increases, in theory, at a ratio of 3:1 (PIM to signal). A 1dB increase in carrier power correlates to a theoretical increase of 3dB in PIM signal power. In practice, the actual effect is closer to 2.3dB as the thermal noise constant -174dBm/Hz becomes an error contributor. This thermal noise floor gets closer to -140dBm as PIM detections/signals are measured in a narrow IF filter, which allows the noise level to increase at a theoretical 10dB/decade. This -140dBm floor is considered a PIM analyzer’s residual IM level.

DIN 7-16 Connectors

The popular DIN 7-16 RF connector was designed to achieve good PIM performance and is regarded as the standard RF connector for engineers who design today’s wireless networks. It has a large contact area enabling the handling of 20 and 40W signal levels with minimal contribution of non-linear PIM energy.

With a PIM specification of better than -122dBm and a return loss of higher than 22dB (up to 3GHz), the DIN 7-16 connector is a trusted ally to today’s network designers. The traditionally popular N connector does not perform as well in the presence of 20W or 40W multi-tones primarily because of ferromagnetic effects. Some connector manufacturers have improved their N connector PIM performance through silver plating techniques, but the DIN 7-16 connector remains the preferred connector for low PIM requirements.

PIM Location is not Distance to Fault

The concept of PIM location is getting more attention lately. Proponents are comparing the feature to that of a DTF (Distance to Fault) measurement feature with the insinuation that it provides similar measurement benefits and value. Although the idea of a feature, which can locate a PIM source within a network’s physical structure, has a lot of appeal, this section intends to show that, for PIM-source identification, the technique has severe limitations and may not provide much value for two key reasons:

  1. Impractical distance resolution – distance resolution is inversely proportional to swept frequency bandwidth
  2. Low PIM power levels limit PIM location assessment consistency.

The concept of DTF is a mathematical conversion of a frequency spectrum response to an equivalent time response using Inverse Fourier Transform (IFT) mathematical modeling. Fast Fourier Transform (FFT) is the opposite, being a conversion from time domain signals to an equivalent frequency domain spectrum.

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About the author

Peter Jackson is EMEA Director for CCI. He has over 25 years’ RF broadcast and telecoms experience, providing innovative solutions through OEM, System Integrator and Product Supplier businesses for the International RF industry.

CCI provides innovative, cost-effective, revenue-increasing RF solutions for cellular infrastructure, including equipment for 2G, 3G and LTE co-location, coverage enhancement, capacity improvement plus portable PIM test equipment.

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