Cavity Filters

7/31/2019 Cavity Filters

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Designing High Performance

Cavity Filters

Authored by:

Devin CrawfordAnsoft Corporation

Ansoft 2003 / Global Seminars: Delivering Performance

Presentation #7


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Link the Power of Circuit andFinite Element Analysis

w Many problems can be broken down intoconstituent parts:w Waveguide and Cavity Filters

w Large Circuits: package board - component

w Any components connected by transmisison lines orwaveguides!

w The divide and conquer strategy leadsto very efficient and accurate solutions.


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Strategy for combining full-wavethree-dimensional solutions

w Determine how best to subdivide largeproblems.

w

Solve the constituent problemsparametrically.

w Construct models from constituent parts.The existence of parametric 3-D models

enables fast design and optimization ofvery large structures!


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What is the Transfinite ElementMethod?

w The Finite Element method is used to solve Maxwellsequations in the volume of arbitrary three-dimensionalstructures.

z

nnn j +=

and characteristic impedance (specifiesa relationship between E and H)

n

n

propagation constant

attenuation

A single port solution (mode) yields a

propagation constant

w The transfinite element methodis used to determine thetwo-dimensional field solution at the port.

z

n eyxEy= ),(E


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Transfinite Element Method in aNutshell

w Generalized s-parameters fully characterize theblack-box behavior of a three-dimensionalstructure.

w Structures that have identical port solutions, butdifferent internal geometries may often beanalyzed separately and joined at the ports.

w The use of generalized s-parameters that weredetermined using the transfinite element

method insures that the fields are matched atthe port boundary.

Lets look at some examples


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The basic approach

1. Select the building blocks (parametric models) for thedesired structure.

2. Verify the technique on a simple, easily verifiablemodel.

3. Define the parameter space: what range of valuesshould the parameters cover?

4. Synthesize and optimize the desired three-dimensionalstructure using parametric models generated fromHFSS.

5. Verify the design using the full-wave solution of theentire structure.


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Example 1: Waveguide Filter

w The waveguide filter is comprised of e-plane irises thathave been characterized parametrically in HFSS.

1

2

3

4

5 Parametric model

based on full-wave

analysis.

),(),(),(),(

2221

1211

dSdSdSdS

S-parameters depend on

frequency and aperture width.

d


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Realization of a Waveguide Filter

1. Define parameterized model by generating a grid of coarsely spacedsolutions in the parameter space.


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2. Create the entire filter from individual components.

The model is fully parameterized!


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Parameterized Port Impedance

w An important aspect of the circuit model is the frequencydependent port impedance:

if(k>kc10,z10,0)

Z10 is the frequency dependent characteristic impedance.


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Realization of a waveguide filter

3. Optimize the filter response byinterpolating existing solutions.

Theaccuracyo

fthefull-wave

analysisisaccessible

byinterpolating

existingsoluti

ons!


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3. Optimization continued: thegoal response for theoptimization can begenerated using the filtersynthesis tool.

Optimized response using interpolation of

coarsely spaced HFSS solutions

Optimizationg

oal


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-50

-40

-30

-20

-10

0

9.5 9.7 9.9 10.1 10.3 10.5

Frequency (GHz)

d

B

After optimization using interpolated HFSS solutions, accuracy can

be improved by Simulating missing solutions


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Comparison between HFSSand Ansoft Designer

-60

-50

-40

-30

-20

-10

0

9.6 9.8 10 10.2 10.4

Frequency (GHz)

-40

-35

-30

-25

-20

-15

-10

-5

0

9.5 9.7 9.9 10.1 10.3 10.5

Frequency (GHz)

HFSS

Designer|S21|

|S11|

4. Improve the solution accuracyby generating exact full-wavesolutions for the optimizedparameter values.


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Example 2: dielectric/coaxialresonator cavity filter

w Waveguide components are generally simplebecause the behavior is accurately described bysingle mode interaction between components.

w How do we approach structures that are not easilydescribed by single mode behavior?

w Use the example of a mixed resonator filter toinvestigate the approach.


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Single Dielectric Resonator

w As usual, take advantage ofthe symmetry when possiblefor full-wave analysis.

E-field

H-field

Perfect E-

symmetry plane


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Analysis of a Single DielectricResonator

w A single resonator cavity is perfect for investigating theproposed approach because solutions can be obtained very fast,and various methods can be used to obtain the sameinformation.

1.Resonant frequency and Q determinedfrom the Eigenmode solver in HFSS.

2.Solve the driven problem with portexcitations (also gives resonant

frequency and Q)

3. Create a parametric model in HFSS andanalyze this model by matching portmodes in Ansoft Designer (the transfiniteelement method).


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Subdivision of DielectricResonators

w Eigenmode solver, driven solution, and transfinite elementanalysis were used to verify the equivalence of the threeanalysis methods.

+=

3-dimensional

Eigenmode solution

Port solution


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Defining Parameter Space

w The equivalent circuit can be equated to physical dimensions byequating resonant frequencies of the two simulations.

Open circuit = perfect magnetic conductor

Short circuit = perfect electric conductor


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Design and Analysis of a 5pole filter

w 3 parametric models are required to createthis filter.


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5 Pole Cavity Filter

w Only 3 Modes are needed to represent the TE01Dielectric resonator mode.

1 2

3

4

1

2

3

4

5

6

7

1

2

3

4

5

6

7

12

3

4

1

2

3

4

5

6

1

2

3

4

5

6

U4Ideal5pole1

Goal Response

5 Pole Filter

Generated in HFSS

Optimization goal is inserted in

the same schematic as the HFSS

subcircuits

Each connection represents

a mode (port solution)


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5 Pole Filter

w Filter synthesis is carried out in Designer, usingparameterized models that were generated in HFSS.

-3

-2

-1

0

1.92 1.94 1.96 1.98 2.00

Freq (GHz)

S21

(dB)

-60

-40

-20

0

1.85 1.90 1.95 2.00 2.05 2.10

Freq (GHz)

Simulate missing solutions

Solution in Designer

when HFSS models

are interpolated


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Optimized 5-pole filter in HFSS and Designer

|H|

Field magnitude

from HFSS at

1.96 GHz

|S21|dB

|S11|dB

Response in HFSS after optimizationin Designer


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Conclusion

w A new approach to filter design and synthesis hasbeen demonstrated.

w The speed of circuit analysis along with theaccuracy and flexibility of the finite elementmethod will bring filter design and synthesis to thenext level.

Cavity Filters

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