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Q: How is PIM Specified?

A: Passive IM is typically specified in absolute power(units of dBm) or power relative to only one of the testtones (units of dBc). For example, a -110 dBm IM signalcaused by two +43 dBm tones is also specified as a -153dBc IM level.

In the case of unequal carrier power levels, SummitekInstruments has established the convention that units ofdBc are relative to the largest of the incident carriers.

It is important to note that a carrier power level mustalways be specified with the given PIM performance level.This applies equally to PIM performance specified in unitsof dBm and dBc.

Q: Does PIM Vary with Power Level?

A: Yes. However, the relationship between the generatedPIM power level and the parent carrier power levels is notalways straightforward.

In broadband devices terminated into a broadbandtermination, the IM3 response typically increasesapproximately 3 dB for every one dB in carrier power level(assuming equal carrier powers). However, there aremany factors which may not follow this rule, including:

High return loss values at n*F1 and/or m*F2. Extreme slope variations on the hysteresis curves

associated with ferrite devices. Non-Linear behaviour of electromechanical junctions

as they approach a breakdown potential , and The interaction of multiple IM sources as the

impedance of each IM source changes with incidentpower level.

In general, as the transmitter power increases, theimportance of PIM on the overall system performancebecomes of increasing concern. As a TDMA system fillsavailable frequency and time channel slots, or as a CDMAsystem increases forward power levels to increasecapacity, PIM levels typically increase.

Q: Are there Measurement Standards for PassiveIM?

A: The International Electro-technical Commission (IEC)

formed a Technical Committee to prepare test methodsand to investigate relevant limits, for PassiveIntermodulation in the RF and microwave frequency rangefor passive components (i.e. connectors, cables, cableassemblies, waveguide assemblies and components...).

The result is IEC62037, an international standard formeasuring PIM. It provides a block diagram for the testapparatus required and defines the test parameters, scopeand techniques that should be used during the process.

The issue of dynamic measurements is given a great dealof attention with regard to cable assemblies, due toconcern over the vulnerability of the connector/cableinterface as well as IM created in the cable (by micro-cracks in solid conductor cables and discontinuous contactin braided cables). Testing involves measuring the IM asthe cable is flexed and/or a bending moment is applied atthe connector/cable interface.

This group has been meeting for several years, andfurther information can be obtained from www.iec.ch.

Q: What is a "Good" PIM Level?

A: Low enough not to affect the performance of the

associated receive if products where to fall in band.The required PIM performance for a given RF device is astrong function of where that device is located in the finalsystem. For example, an antenna must have excellent PIMperformance as the PIM generated in the antenna is bothreceived and radiated by the base station.

Further, the transmit antenna is subjected to nearly thefull carrier power of the base station. On the other hand,the PIM performance of a receive "clean-up" filter neednot be so stringent. This filter might be located on theother side of a diplexer thus preventing the full carrierpower level from reaching its input connector.

Ultimately, it is up to the buyer to specify the maximumacceptable PIM level and carrier power levels. Commonlyseen specifications for antennas are -100 to -110 dBm IM3levels with 2x +43 dBm (20 Watt) per carrier tones.

Q: What are the most common causes of PIM thatyou find?

A: Around 80% of PIM problems are connector related.Many issues come from loose connectors, poorterminations, water ingress, contaminated surfaces, flux,metal fillings, burrs, dirt orother foreign objects,including dead insects or thesecretions thereof, beingfound in connectors.

The need for cleanliness inthe construction of infrastructure cannot be overstated.

Q: Is a PIM measurement better then RL or DTF?

A: No. These are equally importance measurements.PIM measurements confirm the linearity of the RF system.

RL and DTF reflect Z, reactive impedance, and are usedto measure how well each RF component are electrically

matched. A DTF measurement is perfect for finding thedistance to a fault where the fault has changed Z froman original or specified value. A recorded plot can also

PIM Frequently Asked Questions (Part 6 of 6)Using Passive Intermodulation (PIM) measurement as a Quality Control (QC) metric is a relatively new subject to those incellular industry. In discussion with engineers and technicians, a number of questions are commonly asked, and this list ofFrequently Asked Questions is an attempt to provide some of the answers.

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FootprintChanging the RF cost performance paradigm

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provide an excellent reference of the commissionedcondition of an RF system. It should not be used as thefinal acceptance criteria of an RF system.

Q: Can a PIM test reveal defects that will be missed

by measuring DTF?

A: Yes. It is possible a non linear region along the RFsystem can be seen as a change in Z but this is notalways the case. Often a spur, scratch, or introducedcontamination will not affect the measurable level of Z.

Also, installation practice, intentionally or otherwise, mayneutralise (tune out) a high level of Z by adjustingconnector torque. A DTF plot will see this Z as averagedwithin a measurement increment. Both the initial defectand the attempt to correct this will show up non linearityat this point as a level of unwanted prospectiveinterference or a PIM product.

Q: Which connectors do you recommend I use onmy infrastructure?

A: Most reputable connector suppliers have good and badconnectors in their range. We suggest that all connectorsshould be thoroughly assesses, in conjunction with thecable or component they are to be used with.

Where a connector will be used matters and allenvironmental conditions should be considered for theirlong term impact on the connector chosen.

From the aspect of good design and reliability we haverecommend using DIN 7/16 connectors from a reputablesupplier in the majority of cases.

DIN 7/16 connectors typically have lower PIMspecifications, adequate torque levels, and high reliability.

N type connectors are generally harder to consistently

assemble in the field and are subject to loosening overtime.

Crimp connectors are extremely difficult to terminatereliably. These should never be used in the RF transmitpath. Loose joint or braid threads are a PIM source.

Q: How common are PIM problems?

A: Through extensive testing with operators in Australia,Africa, Asia/Pacific, USA and Europe, we estimate morethan 60% of existing sites have some form of PIMproblem.

Q: While PIM testing a multi-coupler, do I need toterminate all ports?

A: You need to terminate all ports passing the testfrequencies a low PIM load.

Note: High power resistive loads typically exhibitvery poor PIM, usually around -80dBc. Use low IMcable type loads (typically