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

Transcription

1 Title: Tuning Methods for Bandpass Filters using CST Studio Suite Solvers 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 CST MICROWAVE STUDIO Mar-09

2 Overview Introduction Design Specifications for a test vehicle Tuning Methods 3D/Circuit Group-delay Port tuning InverseChirpZ Summary 2 CST MICROWAVE STUDIO Mar-09

3 Introduction Classification of Filters LP-Prototype 3 CST MICROWAVE STUDIO Mar-09

4 Typical Flow Chart of the Filter design and Tuning process Specifications Circuit Design Analytical models Empirical adjustments on the structure Measurements 4 CST MICROWAVE STUDIO Mar-09

5 Improved Flow Chart of the filter design and tuning process Specifications OK? + Measurements Circuit Design Corrections - 3D EM Simulation Output Response 5 CST MICROWAVE STUDIO Mar-09

6 Overview Introduction Design Specifications for a test vehicle Tuning Methods 3D/Circuit Group-delay Port tuning InverseChirpZ Summary 6 CST MICROWAVE STUDIO Mar-09

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, ) Group Delay Time t_d1 = 18,153 ns t_d2 = 30,566 ns t_d3 = 51,798 ns t_d4 = 47,057 ns 7 t_d5 = 71,78 ns

8 Eigenmode Analysis Variable Dimensions c Internal Q should be optimized at a given Frequency a Goals: 8 CST UGM Mar-09

9 Single Cavity + Feed S-Parameter? Useful information in the phase 9 CST UGM Mar-09

10 Group Delay Time, external Q and Input Coupling g _ delay External Q Input Coupling (in f-units) 10 CST UGM 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 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 CST MICROWAVE STUDIO Mar-09

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 g _ delay 0! 14 CST UGM Mar-09

15 Groupdelay: Tuning of 1st and 2nd Resonator Only two variables at a time!! 15 CST UGM Mar-09

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 CST UGM 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 17 CST UGM Mar-09

18 Even/Odd Eigenmodes: Tuning of the 3rd Resonator Even-Mode Odd-Mode 18 CST UGM Mar-09

19 Groupdelay: All resonators open 19 CST UGM Mar-09

20 Groupdelay: 2nd Iteration Redo the tuning again, shown here is the 3rd resonator tuning Nearly perfect Perfect, dl(tuner2)= 15 mue-m! 20 CST UGM Mar-09

21 Geometrical Differences between the two Iteration Passes 21 CST UGM Mar-09

22 Accuracy vs. Meshdensity I 22 CST UGM Mar-09

23 Accuracy vs. Meshdensity II 23 CST UGM Mar-09

24 Accuracy vs. Meshdensity III 24 CST UGM Mar-09

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 *) Fast resonant solver Coupl_tuner_23 Re_tuner_L_2 Re_tuner_L_1 Ke_offset 25 CST UGM Mar-09

26 Overview Introduction Design Specifications for a test vehicle Tuning Methods 3D/Circuit Group-delay Port tuning InverseChirpZ Summary 26 CST MICROWAVE STUDIO 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 27 CST UGM Mar-09

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 CST UGM Mar-09

29 Method of Porttuning 1. Coupling between resonators are designed as negative Cs (act as TLs 90 deg) tuned via optimisation (GA: simplex) 29 CST UGM Mar-09

30 Overview Introduction Design Specifications for a test vehicle Tuning Methods 3D/Circuit Group-delay Port tuning InverseChirpZ Summary 30 CST MICROWAVE STUDIO 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 S-Parameter Inverse Chirp-Z response fo ICZ-Bandwidth 31 CST UGM Mar-09

32 Inverse Chirp-Z Transformation Tuning of 1st resonator Tuned to a min.dip 2 1 Tuning of 2nd resonator 32 CST UGM Mar-09

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 CST UGM Mar-09

34 Inverse Chirp-Z Transformation 34 CST UGM Mar-09

35 Introduction of a single Crosscoupling Tuned using the Simplex Optimizer 35 CST UGM Mar-09

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 CST UGM Mar-09

37 Introduction of a single Crosscoupling Resonator 1 Resonator 2 Resonator 3 Resonator 4 37 CST UGM Mar-09

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

43 Thermal Compensation of Cavity Resonators Vratislav Sokol 43

44 Thermal dependence of Resonant Frequency Could Hot L = L0 (1+ α dt), α thermal expansion coefficient α 20e-6/K L df/dt = khz/k 44

45 Simulation in CST MWS All dimensions are defined as a function of temperature. 45

46 Thermal Compensation Idea Al (α=26.0e-6/k) Ms (α=18.4e-6/k) Reduction of capacitance Al Al Without Compensation Compensated 46

47 Optimal gap dimension gap Gap=2.5 mm df/dt = -0.7 khz/k 47

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. 48

49 49 Thank you for your attention