Communications systems designers know all too well about intermodulation distortion (IMD) and how it can degrade performancesometimes even causing the failure of a wireless base station. Active components, such as amplifiers, are typically the source of IMD. But as cellular and wireless transmission systems have moved to higher-order modulation formats and higher transmit power levels, a different form of nonlinear distortion has made its impact on wireless system operation: passive intermodulation (PIM) distortion. Because PIM can originate from components as commonplace as cables and connectors, it can be difficult to isolate and eliminate. Fortunately, wireless on-site engineers and technicians have a new weapon in their war against PIM, in the form of the PIM Masterseries of portable test instruments from Anritsu Co. The firm's models MW8208A, MW8209A, and MW8219A PIM masters are designed for use at specific wireless/cellular transmit frequency bands, such as 869 to 894 MHz and 1930 to 1990 MHz, and can track PIM levels in the associated receive bands where they do the most harm.

The models MW8208A, MW8209A, and MW8219A PIM Masters (Fig. 1) can each generate a pair of continuous-wave (CW) test tones at user-selectable output levels of +43, +45, and +46 dBm (20, 30, and 40 W each) in order to reveal sources of third-, fifth-, and seventh-order IMD produced by passive components or signal junctions in cell sites and wireless base transceiver stations (BTS). the instruments are designed to work with Anritsu's portable, handheld wireless on-site testerssuch as the Site Master, the Spectrum Master, the Cell Master, and the BTS Master instrumentsto form a complete signal stimulus and monitoring solution for specific wireless transmit and receive bands of interest.

For example, the model MW8208A PIM Master has a transmit frequency range of 869 to 894 MHz and a PIM analysis range of 824 to 849 MHz, which is the receive band associated with the 850-MHz cellular band. The model MW8209A PIM master has a transmit frequency range of 925 to 960 MHz and a PIM analysis range of 880 to 915 MHz, or the receive range for 900-MHz E-GSM systems. Finally, the model MW8219A PIM master provides transmit frequency ranges of 1930 to 1990 MHz and 2110 to 2155 MHz, with PIM analysis range of 1710 to 1755 MHz and 1850 to 1910 MHz, which are the receive bands for 1900- and 2100-MHz PCS and AWS wireless systems, respectively.

In addition to generating precise, high-level test signals, the PIM Master instruments provide unique test functionality in their Distance-to-PIM (DTP) capability, which can simply and accurately find the location of PIM sources in a cellular/wireless system, even when that source is outside the path of the system's cables, connectors, and antennas. This DTP functionality is similar to the distance-to-fault capability introduced in Anritsu's Site Master family of portable spectrum analyzers (in 1997). In the case of the Site Masters, the DTF measurements display the distance to a change in impedance or an impedance fault. In the PIM Masters, the DTP measurement shows the distance and relative magnitudes of all PIM sources which are caused by nonlinear phenomena, both generated by the antenna and its cabling as well as outside the antenna, such as a rusty bolt on the antenna tower. The patent-pending DTP technique employs proven algorithms to determine the relative magnitude of PIM sources and the locations of these nonlinear faults.

Each PIM Master measures about 12 x 17 x 20 in. (300 x 425 x 500 mm) and weighs about 59 lbs. (27 kg) to facilitate transport to a wireless or cellular antenna site for testing. In addition, the PIM Masters are available with a Global Positioning System (GPS) option via the handheld instrument to record the precise location of the test site as well as the locations of the measured PIM sources relative to a reference location of choice. The PIM Masters are straightforward to use: Each instrument includes a connection diagram on its front panel, with a test port that connects the two high-power test tones to a base station's transmit antenna. Also included are RF output and reference connector ports that connect to a Site Master or other analysis instrument for monitoring the generated PIM signal products. The PIM Master sends two high-power CW test tones along an antenna feed line, and uses the same cable to measure the amplitudes and locations of the PIM sources as returning signals.

Problems From PIM
So, what is PIM anyway, and why is it such a problem? Although engineers concerned with cellular/wireless site performance initially wrestled with active sources of IMD, including amplifiers, PIM has become a serious problem in recent years due to the growing use of higher-order modulation formats, the use of higher data rates in wireless networks, increasing traffic on each cell site, sites handling multiple carriers, and even performance deterioration of hardware (such as cables and connectors) at wireless sites due to aging. IMD problems due to active components can quickly be isolated, since the location of those components is known. PIM is more difficult to isolate since it can originate almost anywhere along a cable assembly or antenna, and can even originate outside of the transmission system, as the result of signal transmissions reacting with a rusty metal bolt in the antenna tower or some other ferrous component.

When PIM is thought to be the cause of a BTS receiver's deteriorating noise floor or shrinking coverage, the primary suspect as the cause of PIM is usually the coaxial connectors in the transmission lines. PIM is caused by two or more signals that mix in a nonlinear junction, such as a cold solder joint or a damaged coaxial connector. Connectors are usually suspected as being a PIM source because if the center contact is set too deep, insufficient contact is made for a good electrical connection. If the center contact is not deep enough, damage can occur from over tightening the mated parts of a connector. Even the wrong materials, such as nickel, iron, and even stainless steel (if used in the transmission signal path), can generate excessive levels of PIM in a wireless BTS.

As the signals mix in a nonlinear junction, such as a damaged coaxial connector, intermodulation products are formed as the result of the relationship of the mixing signals. If a fundamental, or firstorder, signal interacts with the harmonics, or second-order products, of a second signal, the order of the intermodulation is said to be 1 + 2 = 3 or third-order PIM. The simple equations that are used to show how intermodulation products are generated for two transmit carrier signals, F1 and F2, are:
IM n+m = nF1 mF2
IM n+m = nF2 mF1

The factors m and n are positive integers, with their sum, m + n, representing the order of the intermodulation product, such as 2 + 1 = 3 for third-order intermodulation. The amplitude of PIM tends to decrease with increasing order, with seventh-order PIM typically being at much lower levels than third-order PIM (Fig. 2). But PIM products increase in bandwidth with increasing order, and can contribute significantly to a BTS receiver's noise floor when the PIM products fall within the operating band of the BTS receiver (Fig. 3). With the addition of modulation, the bandwidth of PIM products increases further, increasing as a function of modulation complexity.1

PIM has traditionally been tested by generating two known high-power test signals and monitoring the levels of the predicted PIM signals with a spectrum analyzer. A BTS site can exhibit any number of symptoms as a result of PIM problems, including receiver desensitization (increased receiver noise floor), increased receiver diversity alarms, spectral regrowth in the transmitter mask, an increasing number of dropped or blocked calls, a dramatically reduction in the coverage provided by the cell site, and even complaints of interference from neighboring cell sites.

The PIM Master instruments provide adequate test power, at 2 x 40 W, to locate PIM sources that may be intermittent or in their beginning stages, such as problems in connectors due to microscopic arcing. Test tones can be set for duration of 1 to 60 seconds, and results can be shown on an analyzer screen (Fig. 4) or audible tones can be used to signal relative PIM levels during a measurement. Operators must properly set up the instruments prior to a measurement, in terms of setting the desired test tones, F1 and F2, selecting the intermodulation order (third, fifth, or seventh order), the frequency span to be analyzed for the PIM products, the velocity of propagation for the cable being used in the testing, and the distance of the cable.

Once the measurement setup is established, it can be saved as a setup file and used as a reference for comparison of subsequent measurements. This makes it possible to make a measurement at a site and use it as a reference for later visits to that site, to compare for performance deterioration.

Each measurement can be saved as a measurement file, and even screen shots can be saved as JPEG format graphic files. With the combination of the PIM Master and a portable analyzer such as a Site Master or Spectrum Master, it is possible to perform PIM and line-sweep measurements, the latter which can provide information on the insertion loss and return loss of the transmission path or, essentially, how well that signal path is impedance matched. Both tests are important for checking the overall quality of a cell site's or base station's antenna feed line. More information an be found in a free white paper, "Troubleshooting Passive Intermodulation Problems in the Field," available for download from the Anritsu Co. website.2

Although the PIM Master is relatively simple to use, properly applying the instrument and its companion Site Master or other analyzer for a set of PIM measurements across a given wireless band can require precision and care. To aid engineers and technicians in the use of the PIM Master instrument line, Anritsu offers a PIM Master one-day training course on optimal use of the instrument. It is also available with numerous options, such as the built-in GPS capability via the partnering handheld instrument, which are shown in greater detail on the Anritsu website. P&A: $26,500 and up; 4 to 6 wks ARO.

Anritsu Co.
490 Jarvis Dr.
Morgan Hill, CA 95037-2809
(408) 778-2000
FAX: (408) 776-1744
www.anritsu.com.

References
1. "Understanding PIM," white paper, Anritsu Co., Morgan Hill, CA, 2012, 28 pp.
2. Nicholas Cannon, "Troubleshooting Passive Intermodulation Problems in the Field," white paper, Anritsu Co., Morgan Hill, CA, 2010, www.anritsu.com, Document No. 11410- 00586, p. 3.