Bandpass Tuning methods

1

Title: Tuning Methods for Bandpass Filters using

CST Studio Suite™ Solvers Technology

Company Name:

Name:

CST AG

Franz Hirtenfelder

Job Title:

Department:

Applications Engineer

Sales and Support

Email: franz.hirtenfelder@cst.com

Abstract:

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.

CST MICROWAVE STUDIO ® www.cst.com

Mar-09

2

Overview

• Introduction

• Design Specifications for a test vehicle

• Tuning Methods 3D/Circuit

– Group-delay

– Port tuning

– InverseChirpZ

• Summary


Mar-09

Introduction

Classification of Filters

LP-Prototype

3 CST MICROWAVE STUDIO ® www.cst.com

Mar-09

4

Typical Flow Chart of the Filter design and

Tuning process

Specifications Circuit Design

Analytical models

Empirical adjustments on the structure

Measurements


Mar-09

5

Improved Flow Chart of the filter design and tuning process

Specifications Measurements

+

OK?

-

Circuit Design

Corrections

3D EM Simulation

Output Response


Mar-09

6

Overview

• Introduction



– Group-delay

– Port tuning

– InverseChirpZ

• Summary


Mar-09

7

Cavity Design

Defining the Specifications

Tchebychev 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 www.cst.com

c

8

Eigenmode Analysis

Variable Dimensions a

Internal Q should be optimized at a given Frequency

Goals:

CST UGM 2009 www.cst.com

Mar-09

Single Cavity + Feed

9

S-Parameter ?

Useful information in the phase


Mar-09

10

Group Delay Time , external Q and Input

Coupling g _ delay

External Q

Input Coupling (in f-units)


Mar-09

11

Additional Information about Groupdelay

Coupling Bandwidth,

Group delay

Coupling-Coefficients and Td-Values computations are available via Macro

GroupDelay-Macros and 1D

ResultsTemplates available for CST-

MWS and CST-DS www.cst.com

12

Filter Tuning via Groupdelay: Examples

Tuning of a Dual Mode Filter

Iris Coupled Cavity

Filter

Short

Hairpin

Filter www.cst.com

13

Overview

• Introduction



– Group-delay

– Port tuning

– InverseChirpZ

• Summary


Mar-09

Groupdelay: Determine FlatPhase

1. Short all resonators 2. Move deemebdding distance

3. Untill flat phase is found 4. Rotate focal point to e.g. short

14 g _ delay 0 !


Mar-09

15

Groupdelay: Tuning of 1st and 2nd

Resonator

Only two variables at a time!!


Mar-09

Groupdelay: Tuning of the 3rd Resonator

Difficult to achieve response symmetry

Due to geometrical symmetry only one variable has been left over: the coupling between 2nd and 3rd resonator

(theoretically)

16 CST UGM 2009 www.cst.com

Mar-09

17

Pin-Probes: Tuning of the 3rd Resonator

1. Short out all resonators except the pair considered for coupling

2. Add two small discrete ports to excite the modes

3. Coupling bandwidth


Mar-09

18

Even/Odd Eigenmodes:

Tuning of the 3rd Resonator

Even-Mode

Odd-Mode


Mar-09

Groupdelay: All resonators open


Mar-09

Groupdelay: 2nd Iteration

Redo the tuning again, shown here is the 3rd resonator tuning

20

Nearly perfect

Perfect, dL(tuner2)= 15 mue-m!


Mar-09

Geometrical Differences between the two Iteration Passes


Mar-09

Accuracy vs. Meshdensity I


Mar-09

Accuracy vs. Meshdensity II


Mar-09

Accuracy vs. Meshdensity III


Mar-09

25

Accuracy vs. Meshdensity

Variable / Mesh

Coupl_tuner_23 coarse

7.5 mm medium fine

6.35

6.35

Ke_offset

Re_tuner_L_1

Re_tuner_L_2

5.68

6.107

5.165

5.6

5.8

4.94

5.6

5.85

4.97

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

*) Fast resonant solver

Coupl_tuner_23

Re_tuner_L_2

Re_tuner_L_1

Ke_offset


Mar-09

26

Overview

• Introduction



– Group-delay

– Port tuning

– InverseChirpZ

• Summary


Mar-09

27

Method of Porttuning

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

Discrete Ports are assigned at the

Resonators


Mar-09


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

2. C3..c6 set initially to 0 F and then tuned via optimisation (GA: simplex)

3. Missing coupling leads to a slightly mistuned response


Mar-09


1. Coupling between resonators are designed as negative Cs (act as TLs 90 deg)

29 tuned via optimisation (GA: simplex)


Mar-09

30

Overview

• Introduction



– Group-delay

– Port tuning

– InverseChirpZ

• Summary


Mar-09

31

Inverse Chirp-Z Transformation

The 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

Inverse Chirp-Z response S-Parameter fo

ICZ-Bandwidth

1 2 3 4


Mar-09

32


Tuning of 1st resonator

Tuned to a min.dip

2

1

Tuning of 2nd resonator


Mar-09


Tuning of coupling between

1st and 2nd resonator

2

3

1

Tuned to a best fit in time compared to ref. filter

33

Tuning of coupling between

2nd and 3rd resonator


Mar-09



Mar-09

35

Introduction of a single Crosscoupling

Tuned using the Simplex Optimizer


Mar-09

36


Triplet‘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.


Mar-09

37


Resonator 1

Resonator 2

Resonator 3 Resonator 4


Mar-09

38


Realization

A capacitive cross coupling between reasonators 1-3 is forming a triplet section (1-2-3) producing a transmission-zero below the passband

4

3

1

2 www.cst.com

39


Optimizing the structure using Nelder

Mead Simplex Optimizer only for resonator‘s lenghts

1

2

3

4 www.cst.com

40

Introduction of a single Crosscoupling www.cst.com

41


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)

1

2

3 4 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

42 www.cst.com

43

Thermal Compensation of Cavity Resonators

Vratislav Sokol

44

Thermal dependence of Resonant Frequency

Could

Hot

L = L

0

(1+ α·dT),

α…thermal expansion coefficient

α ≈ 20e-6/K

L df/dT = -19.1 kHz/K

Simulation in CST MWS

45

All dimensions are defined as a function of temperature.

Thermal Compensation Idea

Al (α=26.0e-6/K) Ms (α=18.4e-6/K)

Reduction of capacitance

Al

46

Without Compensation

Al

Compensated

Optimal gap dimension gap df/dT = -0.7 kHz/K

47

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.

48

49

Thank you for your attention…

Bandpass Tuning methods

Thermal Compensation of Cavity Resonators

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Bandpass Tuning methods

Thermal Compensation of Cavity Resonators

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