Modelling cavity filter

temperature drift in CST MWS

Dr Novak Petrovic

September 2009 Abridged

Presentation Outline

• Company background

• Role of CST MWS

– Conventional solutions

– HTS solutions

– Ceramic solutions

• Practical Problems and Current Work

– Definition

– CST MWS solution

• Q&A, further discussion

September 2009 Abridged

Company Background

• Founded in 2002

• “Microwave and Materials Designs”

• New materials for microwave applications in the wireless cellular market

• 16 staff with knowledge of RF and materials technologies

• Venture capital backed

• Pursuing OEM & operator relationships

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Solutions overview

• Cellular wireless infrastructure

• Spectrum re-farming (U900)

• Interference mitigation (Custom)

• Co-siting & co-location (Sharing)

• Low noise solutions (Cryogenic)

• LTE – filtering & range extension

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HTS solutions

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Conventional solutions

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Materials solutions

Novel ceramic materials made in house

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Application of CST MWS

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Design and Characterisation

Coupling matrix

optimisation

Design

Tuning

Characterisation

Matlab / Java

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Problem 1

Filter response change due to temperature change?

Thermal expansion

of metals

Change of dimensions

Change of resonant

frequency

Change of filter

response

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Change in performance

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Change in performance

Keep the drift to

less than 1 GSM ch

Operational temperature

specification: -40 to +85 deg C

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Current approach

1tan2 CZfResonance condition:

fringingplatescrew CCCC

diameterresonator_

metercavity_dialn60coaxZ

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Approximating capacitance

fringingplatescrew CCCC

1tan2 CZf

Partly empirical

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Temperature expansion

Thermal expansion coefficient:Linear:

αL << 1

small ΔT

Area:

αA ≈ 2αL

isotropic

materials

Vol:

αV ≈ 3αL

isotropic

materials

T

L

L

0

1

TV

V

V

0

TA

A

A

0

TL

L

L

0

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Automation of current approach

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Automation of current approach

Only works for

“static” analytic cases,

known impedance

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CST MWS solution

Formulate parameters

as a function

of temperature

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CST MWS solution

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Practical caseSpreadsheet calculation: -0.762 MHz drift, -20.2 ppm/˚C

CST MWS: -0.9 MHz drift, -23.8 ppm/˚C

C ppm/deg 106

1

12drift

Tf

fff

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Measured results

Total temperature drift of -0.9 MHz was measured

(+40 ˚C temperature change, rate of -0.023 ˚C/MHz)

CST MWS works

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Extension to arbitrary shapes

αA ≈ 2αL

isotropic

materials

TA

A

A

0

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Scaling check

Reference objects to cavity

(or fixing screw location)

Area of scaled objects

does check out.

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Other possibilities

Parametrise all features

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Accuracy

Criterion: Establish logical convergence (experiments),

by examining change in error, and referenced to mesh.

35 LPW: 141,120 meshcells, f1 = 946.4 MHz f2 = 945.5 MHz

Also confirmed convergence on basic coaxial case.

25 LPW: 54,400 meshcells, f1 = 941.7 MHz f2 = 940.8 MHz

15 LPW: 17,248 meshcells, f1 = 934.5 MHz f2 = 933.6 MHz

45 LPW: 269,500 meshcells, f1 = 946.9 MHz f2 = 946.0 MHz

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Problem 2

• Current work: Design ceramic filter

– Calculate temperature drift and compensate

• Previous methodology applied

• However, a lot more CST MWS simulations

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General design notes

More precise simulations.

Ceramics changes in addition to metal.

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Manufacturer info

τf

Adjustable

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Standard cavity: temperature

D = 0.5’’

H = 0.2’’

L = 1.05’’

C = 1.5’’

f0 ≈ 4 GHz

T

f

ff

0

0

1

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Composite coefficient

T

1

T

L

LC

1

T

D

DT

H

HL

11

T

f

ff

0

0

1

Dielectric constant

Cavity

Resonator (dimensions)

ppm/deg C

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Extract τε

f0 at amb = 3959.39 MHz

f0 at amb + 60˚ C = function of τε (change of ε with T)

Parametrise:

-dielectric const. change

-resonator dim. change

-cavity dim. change

-safe to ignore holder

Change τε to get quoted τf

T

1

τf = 0

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Conversion

Change τε, calc f0 at

amb & temp, calc τf

T εnomr,newr, 1

C deg 60T

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Extract τε: check

C ppm/deg 3.15ε

1,05.0 ,1 ,2

1 CBA

CLεf4

3 CBA

CLf

ε3

4

CB

A

31

r

2

rr

0

4

553.8

LD

f

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Drift of the design

Representative cavity vs entire filter

Metallic resonator methodology

f0 @ amb = 913.15 MHz

f0 @ amb + 60 ˚C = 913.67 MHz

Sweep τε until design stabilises

C ppm/deg 5.9design

C ppm/deg 0 want We design

MWS

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Sweep desired τε

Work out required τε so dopants can be selected

Change until f0 @ +0 ˚C and f0 @ +60 ˚C are the same

C ppm/deg 7.3

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Convert back to τf in std cavity

C ppm/deg 7.3Want

C ppm/deg 8 into Translates f

C ppm/deg 6 toup Rounded f

C ppm/deg 5.1 Therefore design ORDER !

MWS

f0 @ amb = 913.15 MHz

f0 @ amb + 60 ˚C = 913.23 MHz

C ppm/deg 7.14 Have

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Standard cavity: frequency

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Summary

• Work out tau_epsilon from specs

• Work out expected design drift

• Work out required tau_epsilon

• Translate to tau_f in standard cavity

• Translate to standard cavity frequency

• Order

• Keep working...

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AccuracyCase f0

at amb

MHz

f0

at amb

+60 deg C

τf

ppm/deg C

Δf

MHz

40 LPW

606,528

3953.604 3953.658 0.23 0.054

30 LPW

262,236

3956.217 3956.277 0.25 0.06

20 LPW

75,816

3962.602 3962.675 0.31 0.073

15 LPW

30,400

3972.118 3972.207 0.37 0.089

Used τε = -15.3 ppm/deg C

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Keep working...

Entire filter,

all parametrised...

Spurious coupling

(coupling sign)

Temperature drift

Verification

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Tune and measureTuned with the aid of Mesaplexx Filter Design Tool

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Tuning

Extract coupling matrix

by comparing to model

Note difference to desired,

change (screws) manually,

5 – 50 times (?)

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Accuracy

SPW = 6, MIN = 6

Tetrahedrons: 156,411

Adaptation: 1 @ 897.7 MHz

Accuracy = 1e-6

delta S = 0.01 (both)

SPW = 4, MIN = 4

Tetrahedrons: 41,872

Adaptation: None

Accuracy = 1e-4

delta S = 0.01 ~ 7 min

~ 31 min

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Drift illustration-1.5 ppm/deg C

896.92 MHz

vs

897 MHz

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Relevance

• Turn-around time (12 weeks)

• Qualification of custom-made ceramics

– Q, εr, τε (the difficult trio)

– nonlinear coefficients, complex ε

– cryogenic temperatures (non-linear)

• Tight specification (GSM channels)

• New tuners, dual-mode filters, frequency

scaling ...

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Unexpected twistsDielectric supports contribute as well!

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Parameter extraction

Extraction of material properties by numerical simulation.

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Future work

• Power handling in comblines

– Line integral to calculate voltage

• Thermal runaway in ceramic filters

– Power handling

• Printed filters

• Lumped element extraction

• More macros

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2008 - EDN Innovation AwardsWinner – Best application of RF design

Winner – Best overall project

Thank you!

Contact:

Novak Petrovic

novak.petrovic@mesaplexx.com

npetrovic@gmail.com