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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: [email protected]:
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.
2 CST MICROWAVE STUDIO® www.cst.com Mar-09
• Introduction
• Design Specifications for a test vehicle
• Tuning Methods 3D/Circuit– Group-delay
– Port tuning
– InverseChirpZ
• Summary
Overview
3 CST MICROWAVE STUDIO® www.cst.com Mar-09
Introduction
Classification of Filters
LP-Prototype
4 CST MICROWAVE STUDIO® www.cst.com Mar-09
Specifications Circuit Design
Analytical models
Empirical adjustments
on the structure Measurements
Typical Flow Chart of the Filter design and
Tuning process
5 CST MICROWAVE STUDIO® www.cst.com Mar-09
Specifications
Circuit Design
3D EM Simulation
Corrections
Output Response
OK?
+Measurements
-
Improved Flow Chart of the filter design and
tuning process
6 CST MICROWAVE STUDIO® www.cst.com Mar-09
• Introduction
• Design Specifications for a test vehicle
• Tuning Methods 3D/Circuit– Group-delay
– 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:
a
c
9 CST UGM 2009 www.cst.com Mar-09
Single Cavity + Feed
S-Parameter ? Useful information in the phase
10 CST UGM 2009 www.cst.com Mar-09
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
Short
13 CST MICROWAVE STUDIO® www.cst.com Mar-09
• Introduction
• Design Specifications for a test vehicle
• Tuning Methods 3D/Circuit– Group-delay
– Port tuning
– InverseChirpZ
• Summary
Overview
14 CST UGM 2009 www.cst.com Mar-09
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
15 CST UGM 2009 www.cst.com Mar-09
Groupdelay: Tuning of 1st and 2nd
Resonator
Only two variables at a time!!
16 CST UGM 2009 www.cst.com Mar-09
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
17 CST UGM 2009 www.cst.com Mar-09
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
18 CST UGM 2009 www.cst.com Mar-09
Even/Odd Eigenmodes:
Tuning of the 3rd Resonator
Even-Mode
Odd-Mode
20 CST UGM 2009 www.cst.com Mar-09
Groupdelay: 2nd IterationRedo the tuning again, shown here is the 3rd resonator tuning
Nearly perfectPerfect, dL(tuner2)= 15 mue-m!
21 CST UGM 2009 www.cst.com Mar-09
Geometrical Differences between the
two Iteration Passes
25 CST UGM 2009 www.cst.com Mar-09
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
26 CST MICROWAVE STUDIO® www.cst.com Mar-09
• Introduction
• Design Specifications for a test vehicle
• Tuning Methods 3D/Circuit– Group-delay
– Port tuning
– InverseChirpZ
• Summary
Overview
27 CST UGM 2009 www.cst.com Mar-09
Method of Porttuning
Inital 3D geometry is taken from the 1st iteration of the Groupdelay Tuning
Discrete Ports are
assigned at the
Resonators
28 CST UGM 2009 www.cst.com Mar-09
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
29 CST UGM 2009 www.cst.com Mar-09
Method of Porttuning1. Coupling between resonators are designed as negative Cs (act as TLs 90 deg)
tuned via optimisation (GA: simplex)
30 CST MICROWAVE STUDIO® www.cst.com Mar-09
• Introduction
• Design Specifications for a test vehicle
• Tuning Methods 3D/Circuit– Group-delay
– Port tuning
– InverseChirpZ
• Summary
Overview
31 CST UGM 2009 www.cst.com Mar-09
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
fo
Inverse Chirp-Z response
1 2 3 4
32 CST UGM 2009 www.cst.com Mar-09
Tuning of 1st resonator
Tuning of 2nd resonator
12
Tuned to a min.dip
Inverse Chirp-Z Transformation
33 CST UGM 2009 www.cst.com Mar-09
Inverse Chirp-Z Transformation
Tuning of coupling between
1st and 2nd resonator
12
Tuning of coupling between
2nd and 3rd resonator
3
Tuned to a best fit in
time compared to ref.
filter
35 CST UGM 2009 www.cst.com Mar-09
Introduction of a single Crosscoupling
Tuned using the Simplex Optimizer
36 CST UGM 2009 www.cst.com Mar-09
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.
37 CST UGM 2009 www.cst.com Mar-09
Introduction of a single CrosscouplingResonator 1 Resonator 2
Resonator 3 Resonator 4
38 www.cst.com
Introduction of a single Crosscoupling
RealizationA capacitive cross coupling between reasonators 1-3 is forming a triplet
section (1-2-3) producing a transmission-zero below the passband
1
2
3
4
39 www.cst.com
Introduction of a single Crosscoupling
Optimizing the structure using Nelder
Mead Simplex Optimizer only for
resonator‘s lenghts
1
2
3
4
40 www.cst.com
Introduction of a single Crosscoupling
41 www.cst.com
Introduction of a single Crosscoupling
1
2
3 4
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
43
Thermal Compensation
of Cavity Resonators
Vratislav Sokol
44
Thermal dependence of Resonant Frequency
L = L0 (1+ α·dT),
α…thermal expansion
coefficient
α ≈ 20e-6/K
L
HotCould
df/dT = -19.1 kHz/K
45
Simulation in CST MWS
All dimensions are defined as
a function of temperature.
46
Thermal Compensation Idea
Without Compensation Compensated
Al
Al (α=26.0e-6/K)
Al
Ms (α=18.4e-6/K)
Reduction
of
capacitance
47
Optimal gap dimension
df/dT = -0.7 kHz/K
Gap=2.5 mm
gap
48
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.
49
Thank you for your attention…
