8/14/2019 Footprint Part 3 of 6 Working With PIM

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FootprintChanging the RF cost performance paradigm

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A PIM source is often the root cause of poor receiverperformance, producing undesirable noise rise in spreadspectrum technologies, and receiver blocking ordesensitization in other technologies. The outcomes of thisperformance degradation include:

Dropped calls. Loss of coverage and capacity. High maintenance costs. Customer dissatisfaction, and Lost revenue.

Intermodulation Reviewed

Intermodulation sometimes also referred to asIntermodulation Distortion (IMD) is the manifestation ofunwanted signals produced when two or more signals of

different frequencies, pass through a non-linear elementof an RF system. This non-linear feature may be due toamplification, attenuation or impedance that variesaccording to the magnitude of the voltage or currentpresent in applied signals. Intermodulation can be

categorized as:-

Active Intermodulation - Unwanted RF signalsproduced when two or more wanted signals transit apowered system where distortion, and thereforemixing, occurs due to non-linearity within one ormore of the active elements (e.g a signal amplifier or

some any other active device used to process RFsignals such as a receiver front end or the finalstages of a transmitter). Here the energy in theunwanted signals is produced by the active elements.

Passive Intermodulation (PIM) - Unwanted RFsignals produced when two or more wanted signalstransit or contact any non-linear passive elementwithin the RF path. This includes components ofinterconnecting transmission infrastructure, antennasand any re-radiating elements in proximity to the RFsignal field. Here the energy in the unwanted signalis drawn from the wanted signals.

The Basic Math

In the past PIM has been treated as prospective Rx

interference only. Simply doing the math is the mostcommon way to confirm its relevance to Rx performance.

If the input consists of 2 sinusoidal signals, e.g

x(t) = I1cos(w1t) + I2cos(w2t)

the output signal will be y(t) = A . x2(t).

Expanding this, using well-known trigonometric identities,gives:

In this case, the output contains harmonics as well as sumand difference frequencies.

The traditional rule of thumb is to look for odd-orderproducts (e.g 3rd, 5th , 7th , etc) that may fall in the uplinkor Rx (receive) band, knowing that even-order productswill fall out of band.

IM3 = 2f1 +/- 1f2IM5 = 3f1 +/- 2f2IM7 = 4f1 +/- 3f2, etc

This rule usually remains true as long as the frequencies

forming the products are in-band frequencies.

PIM magnitude is a result of a number of variablesincluding carrier bandwidth, the number of primarysignals, the RF power in these and the non-linearcharacteristic of the passive system element.

Generally for a given non-linear element, PIM levels willincrease with RF power and number of signals present.

If two signals of equal power were to produce PIM at-90dBm then a 1dB increase in RF power can increase theunwanted signal by up to 3dB to -87dBm. If three signalsmix at the same power level this would produce a 6dBincrease in the unwanted signal level.

PIM in the Field

In practice, the transmission system can contain acomplex mix of signals from a single band, multiple bands,different RF protocols, collocated signals and otherexternal interference, which can form a multitude ofpotential intermodulation products where signal path non-linearity exists.

This is further complicated by the location of the PIMsource in the RF chain, making the receiver performancepotentially susceptible to both in-band and out-of-bandproduct

Working with PIM (Part 3 of 6)Part 2 of this series of papers introduced the concept of Passive Intermodulation (PIM) within an RF system being a directreflection of physical construction quality. This paper provides an overview of the PIM RF phenomenon, its causes andprevention.

8/14/2019 Footprint Part 3 of 6 Working With PIM

2/2

FootprintChanging the RF cost performance paradigm

Further information visit our websiteswww.summitekinstruments.com

www.triasx.com

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Figure 1 shows prospective PIM sources within the RFinfrastructure at a typical BTS installation.

Our experience is that greater than 60% of existing siteshave poor receiver performance resulting frominterference from a PIM source, with examples including:

Two in-band common path transmitter carriers aredistorted by the non-linearity of a poorly terminatedconnector (the PIM source), mixing to form an in-band Rx product that raises the noise floor, limitingcoverage and capacity.

A transmitter carrier mixing with a second carrierfrom an adjacent antenna through loose screws of anantenna element (the PIM source), to form an in-band Rx product that limits receiver performance.

If the same transmitter carriers form out-of-bandproducts, it is still possible for these to limitperformance by mixing again to form a receiver in-band IM product. Even the best receiver designs willhave some level of non-linearity with the potential toproduce in-band products from a wide range ofsources including the local oscillator.

Spread spectrum W-CDMA (5MHz bandwidth) at850MHz and narrowband GSM 900MHz (250kHzbandwidth) signals sharing a conductor containing aspur or burr acting as a PIM source, are likely toproduce products that will cause coverage andcapacity degradation across the GSM receiver band.

The best way to remove these problems is to lower orremove the interference. Analysis using PIM as the qualitymetric is the simplest, quickest and most sure way of

achieving this.

Prevention - A System Component andConstruction Quality Matter

As pricing constraints have tightened on purchasing

departments, cheaper system components have prevailed.This practice has proved costly in the long run, oftenrequiring lengthy site analysis and eventually componentreplacement. Cheaper components being over specified,or poorly manufactured and tested are often a PIMsource.

Some commonly found PIM sources include: High power loads - Used in large quantities

throughout indoor systems worldwide, they typicallyexhibit very poor PIM, usually around -80dBc. Theseare best replaced with low IM cable type loads(typically