FilterDesign and Tuning using CST Studio Suite€¦ · Filter Design and Tuning using CST Studio...

CST – COMPUTER SIMULATION TECHNOLOGY | www.cst.com | May-12

Filter Design and Tuning

using CST Studio Suite

Franz Hirtenfelder

Applications Engineer

CST Branch Office Munich

Elsenheimer Strasse 55

D-80687 München (Munich)

Germany

Tel: +49 89 2420 828 101, Mob: +49 170 9160 110, Fax: +49 89 2420 828 198

Email: [email protected], Web: www.cst.com

Abstract: Bandpass filters with small fractional bandwidths are quite challenging to design and tune.

After reviewing various tuning techniques using CST Studio Suite a further approach to compute

coupling factor bandwiths simultaneously is demonstrated. A combination of 3D and circuit models

team up with new built in optimizers to efficiently tune bandpass filters. The new System and

Assembly Modelling (SAM) in CST Studio Suite enables the interdisciplinary usage of coupled EM-

Thermal- Structural and Sensitivity analyses within parameter-sweeps and optimization loops and is

demonstrated on a simple test vehicle. Some accuracy considerations with respect to various meshing

techniques conclude this presentation.

mailto:[email protected]

mailto:[email protected]

http://www.cst.com/


Agenda

Introduction

Tuning Methods

Solver selections, accuracy

System Assembly and Modeling (SAM)

Guidelines and Summary


Specifications Circuit Design

Analytical models

Empirical adjustments

on the structure Measurements

Typical Flow Chart of the Filter design and Tuning process

Introduction: Flow Chart

3D EM Simulation

Dimensioning


Introduction: from equations to a 3D model

3D

o

jjii

ijf

cc

ccbw *


Introduction: from equations to a planar model


Parameterization Definition of Goal function Optimizer Choice

1. Define structure parameters

2. Define parameter ranges

Introduction: General Optimizer Setup


local global

Genetic Algorithm

Particle Swarm Optimization

Nelder-Mead Simplex Algorithm

Trust Region Framework

Interpolated Quasi Newton

Classic Powell

Initial parameters already

give a good estimate of the

optimum, parameter ranges

are small

Initial parameters give a

poor estimate of the

optimum, parameter

ranges are large

x

y Example:

Waveguide Corner Goal:

Minimize S11

x

y

CMA Evolutionary Strategy

Introduction: Optimizers


Classic Powell: Quasi Newton: Nelder Mead

Particle Swarm Genetic Algorithm

Trust Region Framework

(TRF)

Introduction: Optimizers


Nelder Mead: 70 Iterations

TRF: 15 Iterations

Introduction: Convergence Speed


Introduction: Summary

To summarize

• In general many parameters to consider

• Wide parameter ranges necessary in order not to

miss an optimum

-> global optimizer strategies required

A possible solution to speed up the tuning effort

is „pretuning“:

Advantage:

local optimizers can be used in addition


Agenda

Introduction

Tuning Methods





Tuning Methods: Introduction

Classification of Filters

LP-Prototype

90º 90º 90º


Coupling Bandwidth,

Group delay

GroupDelay-Macros and

ResultsTemplates available for CST-

MWS and CST-DS

Coupling-Coefficients and Td-Values

computations are available via Macro

Tuning Methods: Group Delay


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

1

2

dtke

2

..1 ode

ftQ

Only two variables at a time!!

Tuning Methods: Group Delay

90º 90º 90º


Tuning of a Dual Mode Filter Iris Coupled Cavity

Filter

Hairpin

Filter

Short

Tuning Methods: Group Delay Examples


Port 1 is excited

Tuning Methods: Field Strengths


The E-max is recorded vs. frequency. What can be observed is that the

peaks of E-max coinside with the peaks of the transmission-groupdelay,

as expected. The groupdelay may act here as a probability function of

energy: High fields when there is a high chance of energy concentration

Tuning Methods: Field Strengths


Add two small discrete ports or

Face ports to excite the modes

Even-Mode F1 Odd-Mode F2

2

2

2

2

1

2

2

2

1

2

2

2

2

1

2

2

2

1

1

2

2

1

2

1_

SS

SS

FF

FF

S

S

S

ScorrectedCBW

Tuning Methods: Pin-Probes and Eigenmodes

CBW

S1, S2… single cavity modes

Pin Probes:


Discrete Ports or Face-

Ports are assigned at

the Resonators

Tuning Methods: Port-Tuning

Initial 3D results


Use Tuner for C3 and C4 to

manually adjust a good filter

response (resonance-tuning)



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

1. Use Tuner for C34

and C45 to manually

adjust a good filter

response



Correlation to the real Geometry (Space Mapping):

Once the optimum for a certain capacitor has been found, we need to find out their relation to the real 3D geometry.

This is done by slightly changing the 3D model for the given resonator where the capacitor is attached to. Then we

need to retune the circuit again to account for this geometrical change. Now we have two positions in 3D and two

values for the capacitance C of the circuit. With a linear extrapolation we are able to find out the desired

mechanical change to set the capacitance to zero, meaning that the lumped capacitance has no impact anymore and

can be eliminated in the circuit.

Mech. Position p1 Ca= x1 F

Mech. Position p2 Ca= x2 F

p1 p2

Ca@p1 =x1F

Ca@p2=x2F

Ca

p px

0

21

1211

xx

ppxppx



To compute the coupling factors k12,k23,

etc…, the Input and Output converters are

separated from the main part of the filter.

Ports of 50 Ohms are assigned and connected

to each resonator. S-Parameter and Y-

Parameter are computed via a DS task. The

imaginary parts of the Y-matrix are used to

compute the coupling factors simultaneously.

3 4 5 6

k12 k23 k34

Tuning Methods:Tuning via Slope Susceptance


k12

k34 k23

))_().(_(((/)(.2.2

derivYimderivYimsqrYim

df

dB

df

dB

Bk jjiiij

jjii

ij

ij


Templates:


Circuit setup for Coupling Coefficients

To get the coupling coefficients simultaneously the circuit is modified such that each resonator is

attached to a e.g. 50 Ohm port. The resulting Y-Matrix is used to compute all couplings:



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

S-Parameter

ICZ-Bandwidth

fo

Inverse Chirp-Z response

1 2 3 4

Tuning Methods:Inverse Chirp Z


Tuning of 1st resonator

Tuning of 2nd resonator

1 2

Tuned to a min.dip

Tuning Methods:Inverse Chirp Z


Example of a Diplexer: Groupdelay

Rx part is considered,

but not shown here,

posts are short out Red curve: Inital groupdelay for the

first two posts next to the common

port after the separate tuning of TX

and RX

Green Curve: tuned Groupdelay

TX

RX


1D Results > invChiprZ_TX

1D Results > invChiprZ_RX

InvChirpZ transformation is applied to the

open filter and it can clearly be seen that

some of the resonators are not tuned to

their center frequency f1 and/or f2. This

is indicated by the dips of the response.

The coupling seems to be rather ok,

repesented by the time-delay.

The advantage of this method is that the

individual mistuned resonators can be

identified!

2nd resonator mistuned

Example of a Diplexer: InvChirpZ

f1 f2


Agenda

Introduction

Tuning Methods





Time Domain (TD) Frequency Domain (FD)

Eigenmode (E)

Modal (Resonant Fields)

Lossy/Lossless

Solver Selection

Resonant: Fast S-parameter (MOR)

General Purpose

Resonant: S-Parameter, Fields (Modal Analysis)


Accuracy vs. Meshdensity

20/20 30/30

10/10


Re_tuner_L_2

Re_tuner_L_1

Coupl_tuner_23

Ke_offset

Variable / Mesh Coarse 10/10 Medium

20/20

Fine 30/30 40/40 60/60

Coupl_tuner_23 7.5 mm 6.35 6.35 6.35 6.35

Ke_offset 5.68 5.6 5.6 5.45 5.45

Re_tuner_L_1 6.107 5.8 5.85 5.722 5.731

Re_tuner_L_2 5.165 4.94 4.97 4.924 4.942

k23 18.2 18.2 18.2 18.22 18.22

CPU Time (HEX MOR) 26s 129s 485s 18m 60m

Accuracy vs. Meshdensity

k12

k23


FD General Purpose Tet-Mesh

Curved Tetras To avoid Mesh Noise:

•Dummy-elements

•Slicing of tuners

(switching of materials)

•Grid-Movement

•Sensitivity


Agenda

Introduction

Tuning Methods





SAM: Multiphysics Simulation


Coupling between Simulation Projects



Parameter Sweep / Optimization

aperture

S-Parameter (pure EM)

S-Parameter

(including Therm./mech.

Deformation)



Agenda

Introduction

Tuning Methods





Some Guidelines

• Start out with a rather coarse mesh models

• Don‘t use meshadaptation

• Pretuning is helpful to apply local optimizer

strategies

• Select the appropriate solver

• Refine the mesh and retune again

(Parameter ranges are a lot smaller)

until parameter changes are smaller than a

given manufactoring tolerance


Summary

• CAD Modeler easy to use with respect to

parameterization

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

•Various optimizer strategies

•Optimization and parameterization control via

complex post processing templates

•Flexible link to circuit simulator CST- DESIGN

STUDIO including CST- MICROWAVE STUDIO –

submodels

• Various tuning procedures available for a successful

tuning

Date post:	05-Jun-2018
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