Title: Tuning Methods for Bandpass Filters using CST Studio Suite Solver Technology

Transcription

1 Title: Tuning Methods for Bandpass Filters using CST Studio Suite Solver Technology Company Name: Name: Job Title: Department: CST AG Franz Hirtenfelder Applications Engineer Sales and Support 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. 1

2 Overview Introduction Design Specifications for a test vehicle Tuning Methods 3D/Circuit Group-delay Port tuning InverseChirpZ Summary 2

3 Introduction Classification of Filters LP-Prototype 3

4 Typical Flow Chart of the Filter design and Tuning process Specifications Circuit Design Analytical models Empirical adjustments on the structure Measurements 4

5 Improved Flow Chart of the filter design and tuning process Specifications OK? + Measurements Circuit Design Corrections - 3D EM Simulation Output Response 5

6 Overview Introduction Design Specifications for a test vehicle Tuning Methods 3D/Circuit Group-delay Port tuning InverseChirpZ Summary 6

7 Defining the Specifications Tchebychev Filter =================== Order = 4 Bandwidth = 25 MHz (rel. BW=2.3%) Center Frequency = 1100 MHz Passband ripple = 0,01 db (1, VSWR) Return loss = -26,3828 db Normed g values: g1 = 0,7129 g2 = 1,2004 g3 = 1,3213 g4 = 0,6476 g5 = 1,1008 Cavity Design Corresponding coupling coefficients in MHz / (rel): k_e = 35,07 (0, ) k1_2 = 27,03 (0, ) k2_3 = 19,85 (0, ) k3_4 = 27,03 (0, ) k_out = 35,07 (0, ) 7 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

8 Eigenmode Analysis Variable Dimensions c Internal Q should be optimized at a given Frequency a Goals: 8

9 Single Cavity + Feed S-Parameter? Useful information in the phase 9

10 Group Delay Time, external Q and Input Coupling g _ delay External Q Input Coupling (in f-units) 10

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 11

12 Filter Tuning via Groupdelay: Examples Tuning of a Dual Mode Filter Iris Coupled Cavity Filter Short Hairpin Filter 12

13 Overview Introduction Design Specifications for a test vehicle Tuning Methods 3D/Circuit Group-delay Port tuning InverseChirpZ Summary 13

14 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!

15 Groupdelay: Tuning of 1st and 2nd Resonator Only two variables at a time!! 15

16 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

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 17

18 Even/Odd Eigenmodes: Tuning of the 3rd Resonator Even-Mode Odd-Mode 18

19 Groupdelay: All resonators open 19

20 Groupdelay: 2nd Iteration Redo the tuning again, shown here is the 3rd resonator tuning 20 Nearly perfect Perfect, dl(tuner2)= 15 mue-m!

21 Geometrical Differences between the two Iteration Passes 21

22 Accuracy vs. Meshdensity I 22

23 Accuracy vs. Meshdensity II 23

24 Accuracy vs. Meshdensity III 24

25 Accuracy vs. Meshdensity Variable / Mesh coarse medium fine Coupl_tuner_ mm Ke_offset Re_tuner_L_ Re_tuner_L_ Mesh/CPU Time *) 11/26sec 17/129 27/485 Coupl_tuner_23 *) Fast resonant solver Re_tuner_L_2 Re_tuner_L_1 Ke_offset 25

26 Overview Introduction Design Specifications for a test vehicle Tuning Methods 3D/Circuit Group-delay Port tuning InverseChirpZ Summary 26

27 Method of Porttuning Inital 3D geometry is taken from the 1st iteration of the Groupdelay Tuning Discrete Ports are assigned at the Resonators 27

28 Method of Porttuning 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 28

29 Method of Porttuning 1. Coupling between resonators are designed as negative Cs (act as TLs 90 deg) tuned via optimisation (GA: simplex) 29

30 Overview Introduction Design Specifications for a test vehicle Tuning Methods 3D/Circuit Group-delay Port tuning InverseChirpZ Summary 30

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 S-Parameter Inverse Chirp-Z response fo ICZ-Bandwidth 31

32 Inverse Chirp-Z Transformation Tuning of 1st resonator Tuned to a min.dip 2 1 Tuning of 2nd resonator 32

33 Inverse Chirp-Z Transformation Tuning of coupling between 1st and 2nd resonator 3 Tuned to a best fit in time compared to ref. filter 2 1 Tuning of coupling between 2nd and 3rd resonator 33

34 Inverse Chirp-Z Transformation 34

35 Introduction of a single Crosscoupling Tuned using the Simplex Optimizer 35

36 Introduction of a single Crosscoupling 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. 36

37 Introduction of a single Crosscoupling Resonator 1 Resonator 2 Resonator 3 Resonator 4 37

38 Introduction of a single Crosscoupling Realization A capacitive cross coupling between reasonators 1-3 is forming a triplet section (1-2-3) producing a transmission-zero below the passband

39 Introduction of a single Crosscoupling Optimizing the structure using Nelder Mead Simplex Optimizer only for resonator s lenghts

40 Introduction of a single Crosscoupling 40

41 Introduction of a single Crosscoupling 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 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