pim causes and cures by wtg_05122014.pdf

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

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


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


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


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


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

LTE PIM Signature

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


UE:

For low data rate typical required Rx


For high data rate typical required Rx


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

pim causes and cures by wtg_05122014.pdf

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