PIM Distortion Effects and Testing

8/8/2019 PIM Distortion Effects and Testing

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What is Passive Intermodulation (PIM)?

Intermodulation is caused when 2 or more RF carriers are mixed in

a system and form unwanted signals.

When passive components containing non-linear elements are the

source of this interference, it is referred to as:

Passive Intermodulation

or

PIM


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PIM is the result of mixing

f!

"

U

fa

fb

fIM2

fb-fa

2 carriers can create a mixing product.


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f!

"

U

fa

fb

fIM2

fIM3

fIM3fb-fa

fb-fa+f

bfa-fb+fa

But this isnt the only mixing product


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f!

"

U

fa

fb

fIM2

fIM3

fIM3

fIM4

fIM5

fIM7

fIM5

fIM7fIM9

fIM11

fIM13fIM6fIM8

In fact there are many mixing products!


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f!

"

U

fa

fb

fIM2

fIM3

fIM3

fIM4

fIM5

fIM7

fIM5

fIM7fIM9

fIM11

fIM13fIM6fIM8

fb-fa

fa-fb+f

a

A real spectrum looks like this:


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!Multiple carriers can mix and createmultiple intermodulation results.

!The odd order intermodulations are closeto carrier frequencies.

! If there is a 3rd order intermodulation,then there are other IMs as well.

!The IM3 is closest to carrier and hasstrongest IM result.

(That is why the PIM tester checks IM3 results.)


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!PIM products fall in the receive band.!Two or more transmitter channels share a

common antenna.!Transmitter signal levels are high.!Receiver sensitivity is high.!Transmitters and receivers are diplexed

(share a common antenna).

PIM is of particular concern when:


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f!

"

UPIM multiplies bandwidth:

If bandwidth of fa and fb is 1 MHz then

BWIM3 = 3 MHz

BWIM5 = 5 MHz

BWIM7 = 7 MHz

fa fb

fIM3

fIM5

fIM7


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Why is PIM important?

!Number of channels per antenna increasing!Average power per channel increasing!Digital systems more prone to PIM than

analogue systems


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Causes of PIM

!All components, even passive components,are active

!Certain physical phenomena cause passivecomponents to become mixers, modulatorsand frequency multipliers

!These effects can limit a systemsperformance


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Causes of PIM

!Surface Oxides!Loose metal to metal contacts!Use of Ferromagnetic materials!Contaminations, eg solder splatters!Contact between dissimilar metals! Insufficient thickness of plated metal,

causing rf heating

!Too much or too little torque atconnections


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Causes of PIM

Non-Linearities take two different forms:

!Contact Non-Linearity!Material Non-Linearity


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Causes of PIM

Causes of Contact Non-Linearities:

! Junction capacitance due to thin oxidelayer between conductors.

!Impurities on metal surface

!Semiconductor tunnel/schottky effect atpoint of contact.

!Contact resistance caused by twodissimilar metals.


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Causes of PIM

Causes of Material Non-Linearities:

!Hysteresis effect in ferromagneticmaterials (nickel, iron, steel).

!Thermal heating due to poor conductionrate (torque, corrosion, cracks).


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Causes of PIM

Non-linearity in Level:

! Diode effect causes PIM! Corrosion

! Saturation produces PIM! Bad joints

!

Magnetic Hysteresiscreates PIM

! Iron, steel, nickel plating

Non linear parts in the RF

path create PIMLinear components do not

create PIM


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Causes of PIM

Antenna showing oxidation within thepower divider.


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Indicators of PIM in a cellular basestation

! Intermodulation products generatedby Tx signals can interfere in the Rx

band.

!The common result is that theseproducts can over-power receivechannels. Calls are dropped or

channels are believed to be occupied

and not being used by the base

station (loss of air time and loss of

money).


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Indicators of PIM in a cellular basestation

!Cell coverage shrinks. Dropped calls!Data transmission rate drops!RX control loop shows no problem!Antenna sweep detects no issue!RX noise level is high

!PIM creates interference in RX band


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What does a field technician need tofix a PIM problem?

!A PIM problem verification!A portable field test set!A PIM strength indicator

!Visual and audible!Local verification/calibration of PIM test

equipment


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The Solution Boonton PIM 20

! Identifies field PIM problems of passivesystem components like antennas,

feeder cables, connectors, diplexers

and directional couplers.

! Test frequencies from 800 to 2200MHz! Adjustable level test frequencies up to

33dBm (2W)

! Typical PIM measurement sensitivity of-155dBc

! Self calibrating! Small, very portable (8kg)! Battery operated


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Verify PIM

! Portable Go/NoGo checkwith PIM20

! Transmission in yourlicensed band

! Detects 3rd order PIM! Numeric and bar display! Acoustic indicator


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PIM 20 - Features

! Easy, intuitive user interface! Multiple Measurement Readings:

Numerical Display

LCD Bar graph

Green / Red LED bar

! PIM alarm (LED, switch)! Audio tone PIM indicator

Loudspeaker and Jack

! Continuous or30 sec measurement signal

! Battery operated!

Rugged, weatherproof case(IP55 closed lid)


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PIM 20 - Standard Accessories

Power supply / Power cord

Car charger

Adaptor 12V

Accessory pouch

Adapter

7/16 (m) N (f) Test Cable N (m/m) PIM 20


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PIM 20 Optional Items(recommended)

Low PIM cable load (5W)

N-type (f)

Low power

PIM source 7/16(m)

Low PIM cable load (40W)

7/16 (f) N (f)

Adaptors

7/16(f) - 7/16(f)

7/16(f) - N-type (m)N-type (f/f)

Test Cable 3m

7/16(m) -7/16(m) PIM 20


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Can a sweeper detect PIM?

!NO!A Single signal does not create mixing!RF Level is too low!Dynamic range is too low


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Are PIM tester and Sweeper complementary?

Yes

Issue

! Bad VSWR! DTF location! PIM problem! Detect bad solder! Detect broken part! Detect iron in RF path

PIM20

! Yes! No! Yes! Yes! Yes! Yes

Sweeper

! Yes! Yes! No! No! Yes! No


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Do I need 20W carrier power?

!Yes, if you are in R&D or requirecomponent compliance test

!Current test standard is 20W!20 W makes you immobile!20 W is a potential health risk

!No, to detect and solve a PIM problem!PIM problems on 20 W are present on 2W!2 W tester is battery powered!2 W tester is portable and easy to carry!2 W has reduced radiation exposure


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Can I test PIM on other frequencies?

!Yes, frequency has a negligible effect onPIM.

BUT

!You should not transmit outside of yourlicensed band.

!Other parts in TX path can be frequencydependent like diplexer, filter, combiner

and antenna.

Documents

PIM Distortion Effects and Testing