1

Multipaction, Corona and Passive

Intermodulation in passive microwave components

2

Agenda

Mutlipaction

Corona (critical pressure)

Passive Intermodulation

3

Multipaction

4

• What is multipaction?

• What are requirements for multipaction?

• Some underlying physics

• Description of multipaction curves

• General procedure for analysis

• Examples

• Preventions

• Design Limitations and effects

• Testing and detection

OUTLINE

5

• Multipaction occurs in passive microwavecomponents when free electrons arepropelled across a gap by the microwaveelectric field and the transit time of freeelectrons crossing the gap coincides with thehalf period (or odd number of) of theoperating frequency

• Electron strikes the surface and releasessecondary electrons

• The newly released electrons are propelledback across the gap by the microwave electricfield which has changed polarity

• When the number of secondary electronsgenerated (and making it across the gap)exceeds the number of primary electronsstriking the surface, an electron cloud isformed, resulting in arcing which is calledmultipaction

Multipaction

Multipaction-overview

Time: (0 to ) Time: ( to 2)

e-

Secondary

Electrons

E E

E = E0 sin (wt)

6

Requirements for Multipaction

• Surfaces with secondary electron emission coefficients greaterthan 1.0

• Vacuum

• Appropriate combination of gap dimension, frequency, andpower to produce a resonant condition

• Sufficient gap to allow electrons to gain energy crossing the gap

• Appropriate geometries

• Initial electrons to start the process

• Time for the discharge to build up

Multipaction

7

Time to Initiate Multipaction

• Multipaction takes time to develop

– 4ns - at 11.1 GHZ measured

– 20 gap crossings - computer simulation

– 15 cycles - suggested by some space agencies

• Fifteen cycles criterion lacked general acceptance

Multipaction

• Twenty gap crossing is accepted as a standard

8

Electron Tracking

Multipaction has three stages

• Drift

– Due to initial velocity

– Oscillation due to field superimposed on motion

– Terminated by collision with wall (low energy, low yield)

• Synchronization

– Electrons travel out of step with applied field

– Some collisions productive; others not

• Build up

– Electrons in sync with applied field

– All collisions somewhat productive

Multipaction

9

Electron Emission - Yield

• Low energy electrons absorbed

• High energy electrons penetrate too deep – yield low

• Yield greater than one required for multipaction

Multipaction

10

Multipaction Mode

Multipaction

11

Multipaction Modes(Hatch and Williams curves)

• Break down is a function offrequency-gap product

• Multipaction is cut-off below acertain f-d product

• Electrons may take an oddmultiple number of half-cyclesto cross the gap

• Number of half cycles ismultipaction mode number

• Electrons in higher order modestake longer to cross the gap.

Multipaction

12

• Multipaction curves are a function of materials

• Curves are limited to four materials

• Multipaction does not happen below curve

Multipaction

Multipaction Modes(Susceptibility curves)

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• Asymmetric modes3/2 cycle transit one way½ cycle transit other direction other wayNet energy gain each directionIntegral number of cycles for each round trip

• Hybrid modesAlternating between modes every round trip

Eg. 3 cycles, 4cycles, 3 cycles, 4cycles, 3 cycles, 4cycles

Transition between two modes

Multipaction Modes

Multipaction

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Gaps in the Microwave structures

Multipaction

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General Analysis Procedure

1. Determine critical gaps

2. Make assumptions about surface materials if necessary

3. Calculate breakdown voltage from the curves

4. Relate voltage in breakdown region to power level

5. Calculate breakdown power

6. Calculate breakdown margin from the applied power

7. Consider geometry effects

Note: This is the worst case analysis procedure. Alternatively therealistic analysis can be conducted by adding voltage probes inthe critical gaps in the EM model(CST, HFSS) to calculate therealistic voltage across the gaps and then compare it to thebreakdown voltage from the curves.

Multipaction

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Example #1 Transition

Two areas for concern are:

The end of the probe to the waveguide wall (Vg)

The coaxial structure where the probe enters the waveguide (Vc)

Vg

Vw

Vc

Multipaction

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Transition

Coaxial Structure

Hole diameter is .162 in. Centre conductor is .050 in.

gap = .056 in = 1.422 mm

@f = 10.95 GHz, fd = 15.57 GHz mm

For Ag/Au surface, breakdown voltage V = 981 V(from multipaction curves)

Pb = V2/2ZC = 9.62 kW (ZC = 50 )

Multipaction

18

Transition

Probe Structure

Typical gap is .140 in.

gap = .140 in = 3.55 mm

@f = 10.95 GHz, fd = 38.94 GHz mm

For Ag/Au surface, breakdown voltage

V= 3500V(from multipaction curves)

Pb = V2/2ZW = 11.3 kW (ZW = 543 )

Multipaction

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Preventive Measures

Decrease the gap size below multipaction cutoff or increase inthe gap sizes higher than operating power/voltage.

Dielectric filling of the gaps

Pressurization of the microwave components

Avoid sharp edges in the microwave assembly

Changing surface conditions ( for example anodyne Plating)

Multipaction

20

Design limitations due to multipaction

Limits on F*d implies limitations on the dimensions of theassembly

Dielectric filling can cause change in S parameters and thereby achange may be required in the dimensions of the assembly in orderto achieve the S-parameter spec.

Pressurization can increase the risk of corona and PIM

Change in surface conditions will imply change in S-parameters

Multipaction

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Direct consequences

• Generation of noise, harmonics and reflected RF power

• Phase changes in the system

• RF signal absorption and reflection

• Irreversible damage to the RF component and amplifierspowering up the unit

• RF components detuning

• Oxidization and contamination of the RF component

Multipaction

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Detection methods

• Local:

– Electron probes

– fibre optic

– Mass spectrometer

• Global :

– nulling

– Harmonics

– Near band noise

Multipaction

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Multipaction Test setup

LPF BPF1

DUTTemperature

Monitoring

System (T Type

Thermocouple)

PM2PM 1

PM 3

PM 5

PM 4

VACUUM CHAMBER

HPA

SynthesizedSource

Internally

NOTE: Single amplifier configuration could be any acceptable combination of amplifiers to acheive full power

PM1: Main Line forward power (Avg)PM2: Main Line Reverse Power (Avg)PM3: Ring Forward Power (Pk)PM4: Ring Through Power (Pk)PM5: Ring Reverse Power (Avg)

BPF 2LNASPECTRUMANALYZER

Near Band

Noise

LNASPECTRUMANALYZER

3rd

Harmonic

W/G Taper Phased

Adjusted

W/G Short

ElectronProbe Pico

Amp

Meter

Multipaction

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Space environment: Electron seeding

• Initial free electrons required to start multipaction

• The radioactive environment will depend on the location

of the component in the satellite and it’s orbit.

• Densities of 107-108 electron/cm3 with energies of 1 Mev

and higher is considered the case for many system

environments.

• An example of an Electron source Cs-137 used in Multipaction

testing

10 μCu

High energy β particles 1.176 Mev.

Half-live 30.07 years, after 10 years 0.794 of initial value

Multipaction

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Multipaction detection plots

Multipaction

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Multipaction

• RF breakdown phenomena

• Takes place under vacuum conditions only.

• Requires the presence of high electrical fields between surfaces

• Free electrons accelerated by RF voltage to create an electron resonance growth.

•The breakdown level is affected by:

- internal geometry (gap size)

- operating frequency

- applied RF voltage

- surface conditions

Time: (0 to ) Time: ( to 2)

e-

Secondary

Electrons

E E

E = E0 sin (wt)

• Results of the RF breakdown:

- disruption or loss of signal

- permanent damage to component

27

Multipaction

• Prediction:

- breakdown level is a function of “fxd”

- typical calculation is based on Hatch & Williams curve

• Prevention:

- control of frequency-gap product

- avoid sharp edges

- dielectric filling

- pressurization

• Detection:

- near band noise floor

- harmonic detection

- forward/reverse power(nuling)

- electron probe detector

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Corona

29

30

• Caused by the electric field next to an object exceeding thebreakdown value for air (medium)

• The kinetic energy of the electrons exceed the ionization energyof the molecules of the medium.

• Breakdown power depends on the nature and density of thegas(medium).

• Breakdown power is a function of pressure and frequency.Typically critical pressure is found to be equal to the operatingfrequency in Torr

• Gap size, sharp edges

• Testing for corona is similar to multipaction as both cause s-parameter performance changes and near band and harmonicnoise

Corona

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• The voltage at which a spark occurs (known as breakdown voltage) is dependent on the product of air pressure (P) and the separation between the electrodes (d).

• Microwave frequencies alter characteristics

Paschen’s Law

Corona

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Critical pressure-Analysis

Corona

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Preventive Measures

Reduce Gap voltage

Dielectric filling of the gaps

Avoid sharp edges

PressurizationCavity/Resonator Lid

Corona

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Passive IntermodulationPIM

(A multidisciplinary problem)

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Outline

• Introduction

• General review

• PIM Sources

• Design examples

– Waveguide

– Coaxial

• Design Rules / Aspects to consider

• PIM testing

Passive Intermodulation

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Introduction

• Passive Intermodulation (PIM ) is a non linear behaviour that generates “noise”at frequencies which are linear combinations of the input carriers.

• The amplitude of the signal created needs to be typically 170 dBc below thetransmit signals ( ~1x10-9 mW) . If higher, it may interfere with the receive signal.

• Good performance needs to be proven over the satellite operating temperaturein space.

• PIM is not only generated within the microwave components. It can also becreated at other locations of the satellite that may be exposed to electricalcurrents e.g. reflectors, thermal banquets. Spacecraft level testing becomes verycomplex.

• Payloads where the antenna is used to transmit and receive require especialattention to passive intermodulation products generation

Passive Intermodulation

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General Review

• PIM is generated by the device non-linear relationship between current and voltage

•This non-linearity excites the harmonics of the input signals (2f1, 2f2, 3f1,…) as well as the intermodulation products (f1+f2, 2f1+f2,…)

• The issue is typically associated with:

- High transmit power level

- High receive sensitivity

- maintained response over thermals is required

• PIM level difficult to analyze or predict

Example:

fc1=3705 MHz

fc2=4195 MHz

Tx Band=3700 – 4200 MHz

Rx Band=5900 – 6400 MHz

COMUX

F1+F2 3rd 5th 7th 9th 11th

Tx

Rx

Carriers

PIM Products

PIM (dB)

Freq (MHz)

Passive Intermodulation

38

Xpim: product frequency calculation

Fpim= n x F1 + m x F2

m, n are integers

PIM order N= |m|+|n|

Passive Intermodulation

39

PIM sources (metal-metal contact)Inadequate Metal to metal contact pressure.

This is when problems related to:

• Surface flatness

• Surface roughness (contact occurs only at the surface peaks)

• Alignment between parts

• Hardware torque sequence. May create high local stresses

• Use of materials with different CTE(Coefficient of Thermal Expansion). Issue over thermals

• Machining defects nicks, scratches, tooling marks…

• Mechanical design. Part plastic deformation is reached when bolts are at full torque

average

min

max

x microinches (e.g. 8, 16, 32)

filter boby halve

filter boby halve

waveguide

A dimension (e.g. 0.75")

Cu shim 0.003-0.005"

Cavity wall

Filter lid

Passive Intermodulation

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PIM sources (Nonlinear materials)

Nonlinear materials.

• Invar and Nickel are the most typical examples.

• Plating process and thickness specified need to ensure that currents induced by EM Waves propagate through silver only, and not the base material

• > 5 skin depths (δs)

• Steel (fasteners, RF connectors)

2s

c

c = speed of light ~ 3 x 108 (m/s)

μ = permeability of free space = 4π x 10-7 (H/m)

ω = 2πf (Hz)

σ = conductivity of Ag = 6.173 x 107 (S/m)

Band Approx. Freq.

Range (GHz)

Skin Depth δs

(Thou)

UHF 0.2 - 0.8 0.178 - 0.0891

L 1.5 – 1.6 0.065 – 0.063

S 2.0 – 2.7 0.056 – 0.048

C 3.7 – 7.25 0.041 – 0.029

X 7.25 – 8.4 0.029 – 0.0275

Ku 10.7 – 18 0.024 – 0.018

Ka 18 – 31 0.024 – 0.014

Q 44 0.012

Passive Intermodulation

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PIM sources

– Tuning screws

• They are located at high fields and currents regions to allow tunability, contact issue remain betweenscrew and filter body

– Poor solder joints

• If some flux residue (non linear material) is left after cleaning operation

– Solder joints under high stress

• Joint may crack creating a contact problem

– Contamination

• Minute metal particle from helical coils, bolts

• Any other material

– Micro cracks

• When semi rigid cables are bent, it may originate a crack on the internal or external conductors

Passive Intermodulation

42

PIM signal amplitude calculation

PIM & log (PN)

P = input power

N= order (e.g. 3)

• The lower the order of the product the stronger is expected its presence.

• General rule is that the amplitude of the PIM signal is proportional to the amplitude

of each carrier.

– It can be applied when the source is non linear materials

– It is in general wrong when the PIM source is metal to metal contacts (due to

mechanical stresses over thermals)

– Not valid at very high powers, PIM generation tend to saturate

-142

-141.5

-141

-140.5

-140

-139.5

-139

-138.5

-138

20 40 60 80 100 120 140

-145+log10(P**3)

PIM (dBm)

P(W)

Equal power applied

at F1 and F2

Passive Intermodulation

43

Design examples (w/g components)A flange connection is the simplest PIM source in a

system. If not taken into consideration it can be a

significant source of PIM.

•A good performance depends on an evenly distributed high

contact pressure maintained across the thermal profile.

•This can be achieved with high pressure grooves and specific

design on flange hole pattern.

•Soft free oxygen Cu shims (0.003-0.005” thick)

Passive Intermodulation

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Design examples (w/g components to reject PIM)

Sicral 1B X-band OMUX ASSY

-160.00

-140.00

-120.00

-100.00

-80.00

-60.00

-40.00

-20.00

0.00

70

00

70

60

71

20

71

80

72

40

73

00

73

60

74

20

74

80

75

40

76

00

76

60

77

20

77

80

78

40

79

00

79

60

80

25

80

85

81

46

82

06

82

66

83

27

83

87

84

47

85

08

85

68

86

28

86

88

87

49

88

09

88

69

89

30

89

90

Frequency

Iso

lati

on

(d

B)

S11 MUX

S12 CH1

S12 CH2

S12 CH3

S12 CH4

S12 cover filter

S11 cover filter

4 Ch OMUX with an output cover filter. Channel filters are TE112 and cover filter is 10-2 TE101 rectangular cavity that provides at least 45 dB isolation. The passive Intermodulation generated mainly at manifold and interconnecting waveguide flanges is rejected by the output filter.

Rx-band

Passive Intermodulation

45

Design examples (w/g components to reject PIM)

Simple case is a single filter that provides isolation at the PIM frequency band. Generally output BPF (TE101), BSF or LP corrugated filter.

•In general these filters don’t have tuning elements.•They are two pieces of aluminum silver plated

Performance drivers are:

•Contact pressure between filter body halves.

Results from previous studies show that a contact pressure of 10000 psi (lb/in2) is required to suppress PIM

•Output flange connection

•Body halves misalignment•High pressure flange•Mechanical load

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Design examples (w/g components)

• The most complex situation is when a high number of channels OMUX is subjected to a “difficult” PIM performance with no output filter.

• More than 20 w/g flange connections

• 4 hole flanges between channel filter and manifold.

• As OMUX is built from INVAR and interconnecting waveguide is aluminum. Contact pressure at flange connection drops at some point in the thermal profile during testing.

• Aluminum base plate and brackets to mount hardware can “pull apart” the flanges at some point during thermal cycling.

Passive Intermodulation

47

Design examples (coaxial assemblies)

Antenna

Connection J3

Transmit Path J1

Receive Path J2

The problem for coaxial devices becomes much more complex.

– Together with the same issues in waveguide, many other are encountered

– As the frequency is lower skin depth effects, defects on the surface like scratches, silver plating problems will be highlighted during testing.

– Contacts at tuning element become more problematic

– Dielectrics needed to support the striplines or present for other High Power problems may apply stress reducing contact pressure

– Connections of components and connectors became complex

• Solder joints are needed

• Connectors present contact issues and in general have a poor performance unless specifically developed and built for this purpose

Passive Intermodulation

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Design Rules (Electrical)

• When possible (in complex assemblies) review the possibility of having an output filter with known good PIM performance by design. This filter would reduce the PIM level enough to make things easier for other components in assembly mainly during manufacturing and assembly.

• Realize electrical dimensions (e.g. couplings, tuning,…) in a way that mechanical contacts and assembly integration is easier.

• Take into account degradation at contacts between materials with different CTE (e.g. invar-aluminum)

• Ensure adequate thickness of silver layer in drawing. As the frequency is lower this becomes more important. S-band, L-Band, UHF

• Whenever possible avoid tuning elements

• If possible have contact joints in locations where current is low

Passive Intermodulation

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Design Rules (Mechanical)

• Contact grooves, sufficient mounting hardware and configuration to have pressure across the thermal profile

– Filter body halves

– Flanges

– Filter body and lid

– Resonators and filter body

– …

• Avoid deformation of the mating surfaces, structural design needs to consider it (e.g. flanges with adequate thickness).

• Reduce the number of joints

• Avoid non linear materials (e.g. use of BeCu screws to support resonators instead of steel).

Passive Intermodulation

50

Testing: set-up layout

location of

DUT input

interface

hot plate

BPF(F

2)

BPF(F

2)

BPF(F

1)

Cover filter

BB

to isolatate

PIM

BPF\LPF

existing in

lab

(to reduce

the noise)

Al

SA

Dete

ctio

n

dip

lexe

r

Al

Al

H-plane

bend

Brackets

Baseplate not bolted if

possible (use weights)

Cover filter BB

DUT

Cover filter BB

Input filter

DUT has

vertical w/g

input

(circulator)

Al

hot plate

BPF

BPF/ LPF filter

existing from

current X-band

set-up!

Al

BPF

SA

Detection

diplexer

Al

Al

Cover filter BB

Set-up validation is required prior formal

test:

•Detection system works properly

•Set-up meets 5 dB better than

required specification

Set-up adequate PIM

performance validation

Example of configuration

during formal test.

DUT is a 2 channel MUX

with output cover filter

Passive Intermodulation

51

Testing: typical PIM generators

The presence of the tuning screw results on

a local enhancement of the surface current Js

Screw’s contact to threads on waveguide

body remains always hard to control

This is the most typical generator to verify

that detection system is able to “detect” PIM

Circulators are the other type of

PIM

Generators most often used

Passive Intermodulation

52

Test summary report

PIM detect PIM free DUT PIM measured

Passive Intermodulation

53

Passive Intermodulation

PIM summary

• PIM is generated by the device non-linear response

• The issue typically associated with:

- high transmit power level

- high receive sensitivity

- only one antenna (common port) for Tx and Rx

• PIM product frequency is predictable

• PIM level difficult to analyze or predict

• Odd PIM orders are found to be more problematic

Example:

fc1=3705 MHz

fc2=4195 MHz

Tx Band=3700 – 4200 MHz

Rx Band=5900 – 6400 MHz

COMUX

F1+F2 3rd 5th 7th 9th 11th

Tx

Rx

Carriers

PIM Products

PIM (dB)

Freq (MHz)

54

Passive Intermodulation

• Dominant contributors of non-linearity(for passive device):

- similar or dissimilar metal-to-metal joints

- poor mechanical contact

- poor surface finish

- high current density

- temperature variation

- magnetic non-linear effects

Simple design rules to reduce PIM level:

- limit the number of parts in the current path

- ensure adequate contact pressure

- smooth surface finish for connecting parts

- eliminate contaminants in the current path

- adequate and uniform plating thickness

- avoid magnetic materials in the current-carrying-path

55

Thank You