PIM Distortion Effects and Testing
Transcript of 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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PIM is the result of mixing
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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PIM is the result of mixing
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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PIM is the result of mixing
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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PIM is the result of mixing
!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 is the result of mixing
!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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PIM is the result of mixing
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.