Narrowband Combline Filter Design with ANSYS HFSS

Transcription

1 Narrowband Combline Filter Design with ANSYS HFSS Daniel G. Swanson, Jr. DGS Associates, LLC Boulder, CO

2 Introduction N = 6 Inline, Cover Loaded, Combline Filter Single combline filters and combline multiplexers can be found in many wireless systems. Today we will introduce a simple design flow for narrowband combline filters using ANSYS HFSS. This material is suitable for the non-specialist who wants a better understanding of narrowband filter design. Combline Filter Design 2

3 Combline Filter Examples Combline Triplexer Combline Filter Design 3

4 Combline Resonator Loading Tuning screw Tuning screw Typical Resonator Resonator Lumped Loading Resonator Loading Cover Loading We have resonators that are less than 90 long that we resonate with capacitance off the end. Resonator loading is perhaps the most flexible. Lumped loading is used at higher frequencies. Cover loading is typically used at lower frequencies. Combline Filter Design 4

5 Input / Output Coupling Options Metal Disk Tapped Resonator Inductive Loop Capacitive Probe Tapping into the resonator works over a broad range of bandwidths and is quite common. Coupling with an inductive loop near the base of the resonator is another option. Using a capacitive probe is a third option. Combline Filter Design 5

6 Combline Filter Design Flow Estimate order of filter and stopband rejection Build a model of the proposed resonator: Compute available unloaded Q Estimate insertion loss Build Kij design curve Build Qex design curve Build a model of complete filter and apply port tuning Use port tuning corrections to refine filter dimensions Do final simulation of filter with loss: Verify insertion loss in passband Verify rejection in stopbands Combline Filter Design 6

7 Wimax Filter Example Center Frequency: Equal Ripple BW: Rejection: Insertion Loss: Return Loss: Temperature Range: Power Handling: f 0 = 3440 MHz BW = 70 MHz (add 10 MHz for temp) >30 f 0 +/- 80 MHz <1 db at band edges RL > 20 db (should add margin) -30 to +70 deg C < 20 dbm Morten Hagensen, Narrowband Microwave Bandpass Filter Design by Coupling Matrix Synthesis, Guided Wave Technology, April 26, Combline Filter Design 7

8 Wimax Filter Example Combline Filter Design 8

9 Combline Filter Asymmetry or Skewing Combline Filter Design 9

10 Estimating Filter Order N Rejection (db) Rejection RtnLoss S 20log 10 ( S RtnLoss (db) 6 S 1) Stopband Insertion Loss Passband Return Loss Reject Bandwidth Filter Bandwidth 2 Any simple formula that estimates filter order, N assumes the filter is symmetrical. Our 2% bandwidth filter is almost symmetrical and this estimate is probably good enough. For broader band combline filters, we may want to generate a circuit theory model to get a better estimate of stopband performance. Combline Filter Design 10

11 Estimating Filter Order 80 MHz 160 MHz N 20log ( ) 5.33 Combline Filter Design 11

12 Qc of Infinitely Long Coaxial Line For a given dominant dimension D, maximum K and hence maximum realizable Q c is achieved when D/d = 3.6, or is about 77 ohms. r Z 0 Q K f D Collect K data from measured filters [1] Combline Filter Design 12

13 Resonator Design: Zo Wave port defined on top surface Outer: 35 x 35 mm Inner: 10 mm dia Height: Don t care Use HFSS as a 2D cross-section solver 80.5 ohms is close enough to ideal Zo. Combline Filter Design 13

14 Resonator Design: Freq and Qu HFSS Eigensolver No Ports 6mm 20 mm 10 mm 12 mm 35 mm Resonator length = 50 deg Surface of box, resonator and screw assumed to be silver plated. Use 80% of ideal conductivity as a starting point. Use measured data from filters to adjust conductivity in the future. Combline Filter Design 14

15 Chebyshev Lowpass Prototype Chebyshev Lowpass Prototype: db ripple, 20 db return loss, 1.22 VSWR N g 0 g 1 g 2 g 3 g 4 g 5 g 6 g 7 g 8 g 9 g 10 g 1 -g N N is the lowpass or bandpass filter order. The g i s are frequency and impedance scaled values for a lowpass filter with a cutoff frequency of = 1 radian and a return loss of 20 db. Any given passband ripple / return loss level requires a unique table. Other tables are available in the literature or the g i s can be computed. Combline Filter Design 15

16 Midband Insertion Loss Chebyshev Lowpass Prototype: db ripple, 20 db return loss, 1.22 VSWR N g 0 g 1 g 2 g 3 g 4 g 5 g 6 g 7 g 8 g 9 g 10 g 1 -g N Loss( N gi f0 i f0) f Q u 0.27 db Q u is a little optimistic, at the high end of what is possible. Loss will be higher at the band edges. Combline Filter Design 16

17 Dishal s Method As early as 1951, Milton Dishal [2] recognized that any narrow band, lumped element or distributed bandpass filter could be described by three fundamental variables: the synchronous tuning frequency, f 0 the couplings between adjacent resonators, K r,r+1 the singly loaded or external Q, Q ex The K ij set the bandwidth of the filter and the Q ex sets the return loss level. For any narrowband filter (<10% bandwidth) we can compute the required K ij and Q ex from the Chebyshev lowpass prototype. The K and Q concept is universal and can be applied to any lumped element or distributed filter topology or technology [4,5]. Combline Filter Design 17

18 Combline Filter Design 18 Definition of Kij and Qex ) ( f f f BW f f f g g BW g g f f f K BW g g f f g g f Q j i j i ij ex f 1 = bandpass filter lower equal ripple frequency f 2 = bandpass filter upper equal ripple frequency f 0 = bandpass filter center frequency BW = percentage bandwidth g i = prototype element value for element i Note: Equations assume Qu is infinite.

19 Our Filter: N = 6, BW = 2.3% Chebyshev Lowpass Prototype: db ripple, 20 db return loss, 1.22 VSWR N g 0 g 1 g 2 g 3 g 4 g 5 g 6 g 7 g 8 g 9 g 10 g 1 -g N K K K Q 1,2 2,3 3,4 ex BW g g g g0 g BW g BW g 1 2 g BW Combline Filter Design 19

20 Computing Iris Widths and Tap Height Our resonator geometry is now fixed. We have enough Qu to meet the insertion loss goal. We have goals for the Kij s and Qex Now we need to compute the iris widths and the tap height. Combline Filter Design 20

21 Basic Two Resonator HFSS Project Distance between resonators is fixed Iris width controls coupling Some details ignored, like corner radii Lossless model Faster No corrections to Kij Make it parametric for future re-use Lumped ports for tuning in our circuit simulator FEM mesh is not perfectly symmetrical Faster than making geometry changes in the EM model Lumped port Combline Filter Design 21

22 Extracting Coupling Coefficents Port1 R1=50ohm R2=(ZR2) ohm Coupling Rev B 16mm 1_1 2_1 R1=50ohm R2=(ZR2) ohm Port2 1 2 C2 C3 2 1 Loosely couple with transformers. (C1) ff (C2) ff We want to force synchronous tuning. At resonance: mag( im( Y (1,1))) 0 mag( im( Y (2,2))) 0 Combline Filter Design 22

23 Extracting Coupling Coefficients -30 db min Coupling Coefficient Coupling Bandwidth f f 2 2 f 0 f f MHz Combline Filter Design 23

24 Dummy Elements Around The Iris There are many evanescent modes in the iris region. The FEM mesher uses energy balance to refine the mesh. The mesh may be too coarse in the iris region for highest accuracy. Add physical detail in the iris region to force a finer mesh. Only important if you are comparing this simulation to measured hardware. Combline Filter Design 24

25 Coupling With and Without Dummies No Coupling Screw / With Dummies No Coupling Screw +2.2% Coupling Coefficient % +3.1% +3.0% How significant is 3%? Iris Width (mm) Combline Filter Design 25

26 Add Coupling Screw We can include a coupling screw in our model set to a nominal depth. A longer screw increases coupling. Combline Filter Design 26

27 Coupling vs Screw Length & Iris Width % Coupling Coefficient % +31.3% Coupling Screw Len = 10 mm Coupling Screw Len = 5 mm Coupling Screw Len = 0 All With Dummies % +25.8% % +22.8% +20.8% 0.01 We can achieve at least +/- 20% tuning around a nominal 5 mm deep screw Iris Width (mm) Combline Filter Design 27

28 Coupling Curve For 2 mm Thick Wall 2 mm wall 6 mm screw 5 mm deep 2 nd order polynomial coefficients Iris Width K 4103 K 2 Combline Filter Design 28

29 Coupling Curve For 5 mm Thick Wall 5 mm wall 6 mm screw 5 mm deep 2 nd order polynomial coefficients Iris Width K 7273 K 2 Combline Filter Design 29

30 Coupling Coefficients vs Iris Thickness Iris Thickness = 5 mm Iris Thickness = 2 mm Coupling Coefficient Coupling is a function of iris width, height and thickness Iris Width (mm) Combline Filter Design 30

31 HFSS Project for Qex Port 2 Port 1 Port1 1 2 Make the model parametric for future re-use. Tune to center frequency at Port 2. Measure reflected group delay at Port 1. Tap height sets the return loss level of our filter. C2 (C1) ff Combline Filter Design 31

32 Port Tuned Reflected Delay Tap_Height = 5 mm Q ex 2 f (GHz) td (ns) Delay Tap Combline Filter Design 32

33 Qex Data Curve Fit in MathCAD F(x) = a + bx + cx 2 Tap Height Delay Delay 2 Combline Filter Design 33

34 HFSS Model of Complete Filter Fully parametric model About 2 hours to build model Solve time: 2 min 7 sec Quad core i-7 notebook April 2014 ANSYS HFSS 2014 with HPC option Combline Filter Design 34

35 Initial Simulation No Tuning C1 = 0 C2 = 0 C3 = 0 (C1) ff (C2) ff (C3) ff Port Port User defined symbol for S-parameter data (C1) ff (C2) ff (C3) ff Combline Filter Design 35

36 Initial Simulation No Tuning Combline Filter Design 36

37 Symmetrical Tune of Resonators C1 = 30.0 C2 = -3.4 C3 = -5.8 (C1) ff (C2) ff (C3) ff Only tune the resonators, not the couplings. Use symmetry to reduce the number of variables. We can tune this manually, don t need an optimizer. Port1 Port (C1) ff (C2) ff (C3) ff Combline Filter Design 37

38 Symmetrical Tune of Resonators Combline Filter Design 38

39 Full Port Tune with EQR_OPT Note: Units are ff and ph C1 = 29.1 C2 = C3 = C4 = C5 = C6 = C12 = C23 = C34 = C45 = C56 = (C1) ff (C2) ff (C3) ff (C12) ff (C23) ff Port pH (C34) ff Port (C56) ff (C45) ff Dedicated optimizer for microwave filters. It finds an exact equal ripple response. It works on any Chebyshev filter that can be defined in your circuit simulator. (C6) ff (C5) ff (C4) ff Combline Filter Design 39

40 Full Port Tune of HFSS Model EQR_OPT finds a perfect equal ripple response. We are meeting our design goals. Combline Filter Design 40

41 Moving The Tuning Screws Note: Units are ff and ph The largest errors are the first and last resonator tunings. This is a well known characteristic of tapped resonators. We can move the tuning screws in the HFSS model to get a feel for the amount of correction needed. C1 = 29.1 C2 = C3 = C4 = C5 = C6 = Port1 Port2 C12 = C23 = C34 = C45 = C56 = pH (C1) ff (C2) ff (C3) ff (C12) ff 6 (C23) ff (C34) ff 5 (C56) ff (C45) ff (C6) ff (C5) ff (C4) ff Combline Filter Design 41

42 Tuning Results Variable Initial Screw Depths (mm) Initial Tunings (ff) Final Screw Depths (mm) Final Tunings (ff) C C C C C C C C C C C We see strong symmetry in the initial tunings. We see some numerical noise in the final tunings. Combline Filter Design 42

43 HFSS Simulation With Loss Combline Filter Design 43

44 Computing Average Qu Q Q u u 27.3 f (GHz) T Loss(dB) d (nsec) 4878 Combline Filter Design 44

45 Summary Dishal s K and Q method leads us to a simple design flow for narrowband filters. We can modernize the method by using HFSS to build the Kij and Qex design curves that we need. We can then build a complete model of our filter in HFSS, port tune it and get a very good prediction of performance. These virtual prototypes in HFSS avoid the time and expense of multiple hardware prototypes. Experience has shown that we can rely on the HFSS filter model. Combline Filter Design 45

46 References [1] R. Levy, R. Snyder and G. Matthaei, Design of Microwave Filters, IEEE Trans. Microwave Theory Tech., vol. MTT-50, pp , March [2] M. Dishal, Alignment and adjustment of synchronously tuned multiple resonate circuit filters, Proc IRE, vol. 30, pp , Nov [3] M. Dishal, A simple design procedure for small percentage bandwidth round-rod interdigital filters, IEEE Trans. Microwave Theory Tech., vol. MTT-13, pp , Sept [4] J. Wong, Microstrip tapped-line filter design, IEEE Trans. Microwave Theory Tech., vol. MTT-27, pp , Jan [5] D. G. Swanson, Jr., Narrow-Band Microwave Filter Design, IEEE Microwave Magazine, vol. 8, no. 5, pp , Oct [6] D. G. Swanson, Jr., Corrections to Narrow-Band Microwave Filter Design, IEEE Microwave Magazine, vol. 9, no. 1, p. 116, Feb Combline Filter Design 48