Bandpass Tuning methods - CST - Computer … types and implementations of bandpass tuning methods...

1 CST MICROWAVE STUDIO® www.cst.com Mar-09

Title: Tuning Methods for Bandpass Filters using

CST Studio Suite™ Solvers TechnologyCompany Name: CST AG

Name: Franz Hirtenfelder

Job Title: Applications Engineer

Department: Sales and Support

Email: franz.hirtenfelder@cst.comAbstract:

Nowadays filter types consisting of multiple cross couplings, high selectivity, group delay flatness

have to be met in the applications demanded by industry. Although the main theory remains very

solid, a deep comprehension of filter concepts and the improvements of EM simulation tools have

led to significant advances in the design and tuning techniques.

Usually, initial filter dimensions will be relatively poor, since the original design does not take into

account the interactions among resonators and multiple couplings. Ideal circuit models are

approximated by resonating and coupling elements to construct a starting model of the filter. EM

simulation and optimization is then applied to make the response of the realized structure close to

the idealized circuit response. Several types and implementations of bandpass tuning methods are

described and applied in this article.

• Introduction

• Design Specifications for a test vehicle

• Tuning Methods 3D/Circuit– Group-delay

– Port tuning

– InverseChirpZ

• Summary

Overview

Introduction

Classification of Filters

LP-Prototype

Specifications Circuit Design

Analytical models

Empirical adjustments

on the structure Measurements

Typical Flow Chart of the Filter design and

Tuning process

Specifications

Circuit Design

3D EM Simulation

Corrections

Output Response

+Measurements

Improved Flow Chart of the filter design and

tuning process

• Introduction

– Port tuning

– InverseChirpZ

• Summary

Overview

7 www.cst.com

Defining the SpecificationsTchebychev Filter

===================

Order = 4

Bandwidth = 25 MHz (rel. BW=2.3%)

Center Frequency = 1100 MHz

Passband ripple = 0,01 dB (1,100747 VSWR)

Return loss = -26,3828 dB

Normed g values:

-------------------------------------------

g1 = 0,7129

g2 = 1,2004

g3 = 1,3213

g4 = 0,6476

g5 = 1,1008

Corresponding coupling coefficients in MHz / (rel):

-------------------------------------------

k_E = 35,07 (0,0318809)

k1_2 = 27,03 (0,0245688)

k2_3 = 19,85 (0,0180464)

k3_4 = 27,03 (0,0245688)

k_out = 35,07 (0,0318809)

Group Delay Time

----------------

t_d1 = 18,153 ns

t_d2 = 30,566 ns

t_d3 = 51,798 ns

t_d4 = 47,057 ns

t_d5 = 71,78 ns

Cavity Design

8 CST UGM 2009 www.cst.com Mar-09

Eigenmode AnalysisVariable Dimensions

Internal Q should be optimized at a given Frequency

Goals:

Single Cavity + Feed

S-Parameter ? Useful information in the phase

Group Delay Time , external Q and Input

Coupling

Input Coupling (in f-units)

External Qdelayg _

11 www.cst.com

Coupling Bandwidth,

Group delay

Additional Information about Groupdelay

GroupDelay-Macros and 1D

ResultsTemplates available for CST-

MWS and CST-DS

Coupling-Coefficients and Td-Values

computations are available via Macro

12 www.cst.com

Tuning of a Dual Mode Filter

Filter Tuning via Groupdelay: Examples

Iris Coupled Cavity

Filter

Hairpin

Filter

• Introduction

– Port tuning

– InverseChirpZ

• Summary

Overview

Groupdelay: Determine FlatPhase1. Short all resonators 2. Move deemebdding distance

3. Untill flat phase is found

!0_ delayg

4. Rotate focal point to e.g. short

Groupdelay: Tuning of 1st and 2nd

Resonator

Only two variables at a time!!

Groupdelay: Tuning of the 3rd Resonator

Due to geometrical symmetry

only one variable has been left

over: the coupling between 2nd

and 3rd resonator

(theoretically)

Difficult to achieve response symmetry

Pin-Probes: Tuning of the 3rd Resonator1. Short out all resonators except

the pair considered for coupling2. Add two small discrete ports to

excite the modes

3. Coupling bandwidth

Even/Odd Eigenmodes:

Tuning of the 3rd Resonator

Even-Mode

Odd-Mode

Groupdelay: All resonators open

Groupdelay: 2nd IterationRedo the tuning again, shown here is the 3rd resonator tuning

Nearly perfectPerfect, dL(tuner2)= 15 mue-m!

Geometrical Differences between the

two Iteration Passes

Accuracy vs. Meshdensity I

Accuracy vs. Meshdensity II

Accuracy vs. Meshdensity III

Re_tuner_L_2

Re_tuner_L_1

Coupl_tuner_23

Ke_offset

Variable / Mesh coarse medium fine

Coupl_tuner_23 7.5 mm 6.35 6.35

Ke_offset 5.68 5.6 5.6

Re_tuner_L_1 6.107 5.8 5.85

Re_tuner_L_2 5.165 4.94 4.97

Mesh/CPU Time *) 11/26sec 17/129 27/485

Accuracy vs. Meshdensity

*) Fast resonant solver

• Introduction

– Port tuning

– InverseChirpZ

• Summary

Overview

Method of Porttuning

Inital 3D geometry is taken from the 1st iteration of the Groupdelay Tuning

Discrete Ports are

assigned at the

Resonators

1. Deembedding of Selfinductance and Selfcapacitance of discrete Ports via macro

Method of Porttuning

2. C3..c6 set initially to 0 F

and then tuned via

optimisation (GA: simplex)

3. Missing coupling leads to a

slightly mistuned response

Method of Porttuning1. Coupling between resonators are designed as negative Cs (act as TLs 90 deg)

tuned via optimisation (GA: simplex)

• Introduction

– Port tuning

– InverseChirpZ

• Summary

Overview

Inverse Chirp-Z TransformationThe chirp Z-Transformation can be used as a more flexible means to calculate discrete

Fourier transforms. In particular, the unit circle version (known as chirp-transform) can be

used to create a high-quality zoom function.

Golden (reference) Filter required

S-Parameter

ICZ-Bandwidth

Inverse Chirp-Z response

1 2 3 4

Tuning of 1st resonator

Tuning of 2nd resonator

Tuned to a min.dip

Inverse Chirp-Z Transformation

Tuning of coupling between

1st and 2nd resonator

Tuning of coupling between

2nd and 3rd resonator

Tuned to a best fit in

time compared to ref.

filter

Introduction of a single Crosscoupling

Tuned using the Simplex Optimizer

Introduction of a single CrosscouplingTriplet‘s resonators have slightly different resonant frequencies

Thus prior to tuning the dips to ist minima, the ICZ center frequency fo needs to be

readjusted. If the readjustment is not performed, the tuning solution is not unique.

Introduction of a single CrosscouplingResonator 1 Resonator 2

Resonator 3 Resonator 4

38 www.cst.com

RealizationA capacitive cross coupling between reasonators 1-3 is forming a triplet

section (1-2-3) producing a transmission-zero below the passband

39 www.cst.com

Optimizing the structure using Nelder

Mead Simplex Optimizer only for

resonator‘s lenghts

40 www.cst.com

41 www.cst.com

Applying the ICZ to the tuned 3D Filter for various „fo“ found by the golden filter

(fo is varied to check that for individual resonators the dip is shwoing a minimum)

42 www.cst.com

Summary

• CAD Modeler easy to use with respect to

parameterization

•CST Complete Technology™: TD, FD, E, Th

•Optimization and parameterization control via

complex post processing templates

•Various meshing techniques available

•Flexible link to circuit simulator CST- DESIGN

STUDIO including CST- MICROWAVE STUDIO –

submodels

• Various tuning procedures available for a successful

tuning

Thermal Compensation

of Cavity Resonators

Vratislav Sokol

Thermal dependence of Resonant Frequency

L = L0 (1+ α·dT),

α…thermal expansion

coefficient

α ≈ 20e-6/K

HotCould

df/dT = -19.1 kHz/K

Simulation in CST MWS

All dimensions are defined as

a function of temperature.

Thermal Compensation Idea

Without Compensation Compensated

Al (α=26.0e-6/K)

Ms (α=18.4e-6/K)

Reduction

capacitance

Optimal gap dimension

df/dT = -0.7 kHz/K

Gap=2.5 mm

Mesh setting issue

2 meshlines over the gap

The number of meshlines over the gap should be kept over the whole

temperature range. Otherwise the frequency jumps might appear.

Thank you for your attention…

Bandpass Tuning methods - CST - Computer … types and implementations of bandpass tuning methods...

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