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PIM Passive Intermodulation Causes, Effects, Measurement and Cures
Murat Eron
May, 2014
page 1
Outline
What is PIM and what causes it?
Why is it important and how important?
Component selection and criteria
Design and specification guidelines
Testing and measuring PIM
Best practices for low PIM performance
page 2
What is PIM?
Passive InterModulation is a non-linear
phenomena
Imperfections in metal contacts or even
surfaces near high power RF may cause
interfering signals (IM products) to appear
in the Rx band of interest
Intermodulation products can occur even
when there is only one broadband carrier
page 3
page 4
F1 F2
F1 F2
F1 F2
RF signal in
RF signal reflected : Goes back thru Rx chain unfiltered!
What is PIM?
RF signal through
Signals of interest
(carriers)
Intermodulation products
filters, connectors,
antenna, etc.
What is intermodulation?
V1(f1)+V2(f2) applied to a nonlinearity
produces frequency components:
2nd order f1+f2, f2-f1
3rd order 2f1-f2, 2f2-f1
4th order 2f2+2f1, 2f2-2f1
5th order 3f1-2f2, 3f2-2f1
Etc.
page 5
Near F1 and F2!
v1(t)+v2(t) av1(t)+bv2(t)+IM(t)
Various IM products
Where is the nonlinearity?
Poor contact surfaces
Poor contact pressure
Dissimilar metals in conduction path
Ferromagnetic materials
Poor plating, surface quality and finish
Poor crimping or cold solder joints
Steel or rusty surfaces near the
antenna
page 6
V
I
The problem
page 7
F1 F2
7th orders
5th orders
3rd orders
Tx Band Rx Band
Interference in
Rx band
Odd order IM
products fall
very near the
carriers,
sometimes
within the Rx
band of the
same or another
operator! Can
not be filtered!
IM bandwidth
is n (order)
times the
bandwidth of
the
fundamentals!
F1 & F2 => carriers (non-CW)
Why is it a problem?
page 8
LTE Data Rates, 20 MHz Bandwidth, 2x2 MIMO, Pedestrian
0
10
20
30
40
50
60
70
80
90
0 5 10 15 20 25 30 35
Mbps
Signal to Noise Ratio (SNR) dB
QPSK
16 QAM
64 QAM
How big is the problem?
page 9
F1 F2
Tx Band Rx Band
Interference in
Rx band
43 dBm carriers
-57 dBm PIM
Almost 30 dB
stronger than the
typical desired Rx
signal for a severe
PIM problem
100 dBc
How big is the problem?
page 10
F1 F2
Tx Band Rx Band
Interference in
Rx band
50 dBm carriers
-90 dBm PIM
Stronger than the typical
desired Rx signal even
for a good PIM
performance when input
power is very high !
140 dBc
How big is the problem?
page 11
F1 F2
Tx Band Rx Band
Interference in
Rx band
Same carriers
power backed off:
40 dBm carriers
-120 dBm PIM
10 dB backoff
improves PIM by about
30 dB 3rd order (in theory!)
160 dBc
How big is the problem?
page 12
F1 F2
Tx Band Rx Band
Interference in
Rx band
30 dBm carriers
-95 dBm PIM
If PIM performance is
bad, even operating
at very low power
may not totally solve
the problem
125 dBc
How big is the problem?
page 13
F1 F2
Tx Band Rx Band
Not all PIM is harmful
How does PIM manifests itself?
Poor signal quality
Excessive noise in Rx band
Interference in adjacent channels
Receive sensitivity degradation
Cell coverage shrinks
Data rates drop, capacity shrinks
Dropped calls
Temperature sensitive interference
page 14
Broadband Carrier PIM Generation
page 15
page 16
LTE PIM Signature
page 17
PIM and noise floor rise
PIM sources
Mentioned nonlinearities manifest
themselves for four reasons:
Poor workmanship
Poor component selection
Lack of required specification
Poor RF planning and design
page 18
Poor workmanship
Connectors not torqued properly, making
poor contact : cause of majority of PIM
problems in the field!
Center pins of connectors misaligned or
bent or not the proper length
Dirty mating surfaces or metal flakes
Mating surfaces not smooth or poor plating
Poor solder joints, cracks, voids or gaps
Loose screws
page 19
PIM sources
PIM sources
Poor component selection
Non-PIM certified components or signal
conditioners between the BTS and DAS
Critical components: Jumper cables, Bias-Ts,
arrestors, connectors/adapters, filters and
duplexers
Connectors with Ni and/or steel content
Crimped cables, SMA or and right angle
connectors
Ferrites in the RF power path
PCBs and housings not designed for low PIM
page 20
PIM sources
Lack of required specification
PIM should be specified where it matters
It matters at the output of the BTS most, less
as loss builds in the RF path
PIM can be a problem even at low power if
there is excessive corrosion or very loose
connections
Across the board specification or over-
specification can be costly
PIM is typically generated in the Tx path only
page 21
PIM sources
Poor RF planning and design
Spatial and Site Planning: Even with perfect components, antennas near mass of steel or
rusted surfaces, bouncing multiple carriers, will generate PIM
Co-siting of narrow band and wide band nodes
Frequency Planning: Certain band allocations in same geography increase chance of
PIM problems. Identify early.
Carrier collaboration can be helpful
Design: Deploy as low RF power as system design requires
Keep each carrier separate as much as possible
page 22
Specifying PIM: dBm or dBc?
dBc has a meaning only in the context of a well
defined upper end (Pout), it is a relative measure
dBm is power per total signal channel bandwidth
page 23
F1 F2 X dBm
Y dBm
(X-Y) dBc
(Absolute vs. relative)
Rx Tx
PIM
Receiver Sensitivities (for reference)
NodeB:
For low data rate typical required Rx
sensitivity: -124 dBm
For high data rate typical required Rx
sensitivity: -115 dBm
UE:
For low data rate typical required Rx
sensitivity: -119 dBm
For high data rate typical required Rx
sensitivity: -95 dBm
page 24
Typical Indoor DAS RF Planning
Guideline used by system integrators:
Maintain -85 dBm for low data rate @ 90%
coverage
Maintain -75 dBm for high data rate
-95 dBm @ 99% coverage for public
safety
page 25
Irrespective of technology and band
How to Estimate PIM?
You can not!
Very weak nonlinear phenomena
Distributed in nature
Result of manufacturing and assembly and
installation imperfections not repeatable!
No good models exist
It is a cumulative effect
page 26
PIM can only be measured in practice
Specifying PIM
page 27
F1 F2 Pout (dBm)
Max PIM (dBm)
dBc Rx (dBm)
Margin
PIM level (always present) should be less than the Rx
sensitivity required in the system (by some margin)
For each dB increase in carrier powers, PIM goes up by 3dB
(ideally!)
Noise floor
Ideally two WCDMA/LTE/GSM carriers centered at F1 and F2:
Specifying PIM
page 28
F1 F2 43 dBm
Max PIM
-140 dBc
or less NodeB Rx and UE
Sensitivity Range
(for reference only)
Noise floor
-97 dBm
or less -124 dBm
~ -75 dBm
Complex modulated
broadband signal
(NOT CW!)
More practical: two CW test tones at F1 and F2:
(typ. system
spec)
PIM
A single wideband carrier can produce PIM
in the Rx band also!
PIM: Broadband
page 29
Rx Tx
PIM in Rx channel PCS A Block 10 MHz carrier
IM3
10 MHz 20 MHz
IM5
IM7
Test Frequencies (Intra-Licensed Band)
page 30
Band Frequency 1 Frequency 2
700 L 728 MHz 746 MHz 710 MHz (IM3)
700 U 746 MHz 763 MHz 780 MHz (IM3)
850 869 MHz 896 MHz 842 MHz (IM3)
PCS 1930 MHz 1990 MHz 1870 MHz (IM3)
AWS 2010 MHz 2155 MHz 1720 MHz (IM5)
Tx carriers/tones IMD Product in
Corresponding Rx band
Recommended for an operator: guard band frequencies
North America
Test Frequencies (Intra-Licensed Band)
page 31
Band Frequency 1 Frequency 2
900 925 MHz 960 MHz 890 MHz (IM3)
1800 1805 MHz 1880 MHz 1730 MHz (IM3)
UMTS 2110 MHz 2170 MHz 1930 MHz (IM7)
2600 2600 MHz 2690 MHz 2550 MHz (IM3)
Tx carriers/tones IMD Product in
Corresponding Rx band
Recommended for an operator: guard band frequencies
Middle East North Africa Europe
Test Frequencies (Inter-Licensed Bands)
Case 1 (Two GSM carriers)
Tx1: 935 MHz & Tx2: 960 MHz
PIM: 910 MHz (Rx Band: 890-915 MHz)
Case 2 (PCS and AWS carriers)
Tx1: 1940 MHz & Tx2: 2130 MHz
PIM: 1750 MHz (Rx Band: 1710-1755 MHz)
Shared components, i.e. DAS equipment,
cables, duplexers, antennas, will generate
inter-licensed-band PIM
page 32
Where to Test? Typical Cell Site
page 33
High power duplexer
BTS
Jumper cables
Bias-T
Surge protector
TMA
Antenna
Tx
Rx
Note: PIM
generated
here will not
get into Rx!
DAS Distributed Antenna Systems
page 34
Where to Test? Typical DAS Interface
page 35
(2 x KM-B99)
PIM testing
should only be
conducted on
lines and
components
carrying (high
power) Tx
signals Microlab
DCC
(DAS Carrier Conditioner)
>60W each Tx
Acceptance Criteria (Typical)
Two carriers, 20W each (per IEC 62037) Lower power for trouble shooting
Av. power measurement
Cover all bands present in the system
Dynamic and static testing
-97 dBm (-140 dBc) Max for typ. system
-110 dBm (-153 dBc) for most
components and cables
(Less stringent for older and active assemblies)
page 36
PIM vs RF Power
page 37
Two different devices tested for PIM vs power:
PIM does not follow expected power law
PIM vs RF Power
page 38
PIM does not follow expected power laws
Noise
floor
PIM vs RF Power
page 39
Receivers are sensitive to absolute power levels
regardless of the input test tone power levels
PIM Testing
page 40
~
~ Low PIM
load DUT
Tx
Rx
F1
F2= F1+F
Duplexer High
power
CW
sources
Detect and display
(reverse PIM)
Component or
DAS Carrier
Conditioner
(DCC)
Reverse PIM (reflected) is
standard measurement
Forward PIM
700 MHz U Band PIM Test
page 41
PIM Testers
page 42
2-20W
Single band
Portable or desk-top
PIM Testing
page 43
Broadband DCS combiner PIM performance
page 44
Low PIM Microlab components
page 45
Broadband reactive
low loss tappers
Low loss hybrids
Low loss duplexers
Low PIM Microlab components
page 46
DC Blocks
Low loss couplers High power terminations
and attenuators
PIM specs vary by product
page 47
Guaranteed PIM less than
Typical PIM less than
Attenuators -153 -160
Cables -155 -160
DAS Carrier Conditioner (Tx inputs)
-153 -156
DC Blocks -150 -155
Diplexers -153 -158
Duplexers -150 -155
Hybrid Combiner -153 -158
Hybrid Couplers -153 -158
Quadraplexers -150 -153
Tappers -153 -158
Terminations (cable) -160 -165
Triplexers -150 -153
Rapid change with technology and process!
Low PIM DAS Carrier Conditioners (DCC)
page 48
High power combiner for multi-band DAS Dual-duplex DAS signal conditioner
Optimum Torque for (less) PIM
page 49
Clean the RF connectors, mating surfaces before use Care should be taken to ensure the connectors are aligned when interfacing Be sure that the connector is fully seated before tightening the coupling nut. Tighten the locking nut by hand initially, and then only do a final torque using a wrench. Remove o-rings from all test equipment adapters and test leads. This will reduce the torque required to achieve a tight, low PIM connection during test and extend the life of the connectors. (Do not remove o-rings from the site jumper cables.) Torque the 7-16 connector to a maximum of 25 N-m using a calibrated torque wrench. Do not allow the body of the connector to rotate while tightening. Keep protective caps installed on RF connectors whenever they are not in use. RF connectors have a finite life, typically rated for 500 mate / de-mate cycles by connector manufacturers.
Connector Care