HFSS和serenade滤波器设计.docx

1、HFSS和serenade滤波器设计Efficient Design of Chebychev Band-Pass Filters with Ansoft HFSS and SerenadeTutorialDr. B. Mayer and Dr. M.H. VogelAnsoft CorporationContentsAbstract 31. Introduction 32. Circuit Representation of the Filter 53. Relationships between Circuit Components and Physical Dimensions in t

2、he Microwave Filter 104. Initial Filter Design in HFSS 145. Curve Fitting in Serenade 166. Corrected Filter Design in HFSS 207. Additional Information from the 3D Field Solver 217a. Effects of Internal Losses 217b. Maximum power-handling capability 237c. Mechanical Tolerances 24References 25Appendix

3、 A Derivation of the Circuit 26Appendix B The Physical Meanings of K and Q 43AbstractAn efficient method is presented to design coaxial Chebychev band-pass filters. The method involves a 3D full-wave field solver, Ansoft HFSS, teaming up with a circuit simulator, Serenade. The authors show how for a

4、 practical case, a 7-pole band pass filter with a ripple of only 0.1 dB, an accurate design is obtained in a matter of days, as opposed to weeks for traditional methods.The method described is also applicable to even more challenging designs of elliptic filters and phase equalizers realized in diele

5、ctric, waveguide or coaxial technology.1 IntroductionIn this paper, we will describe an efficient method to design a filter. The method involves a 3D full-wave field solver teaming up with a circuit simulator. The basic idea has been explored by others 1 but a different circuit was used in the circu

6、it simulator. We will explain our procedure by presenting in detail how we design a Chebychev band pass filter with the following specifications: Center frequency 400 MHzRipple bandwidth 15 MHzRipple 0.1 dBOut-of-band rejection 24 dB at 390 MHz and at 410 MHzIn order to achieve the out-of-band rejec

7、tion, we will need seven poles. The desired filter characteristic is shown in Fig. 1.Fig. 1 Desired filter characteristicAs the basic geometry for this filter we have chosen a cavity with seven coaxial resonators, as shown in Fig. 2. In the figure, the “buckets” have been drawn as wire frames for cl

8、arity, to show that the cylinders dont extend all the way to the bottom.Fig. 2 Basic filter geometry This geometry is symmetrical with respect to the central cylinder. In this kind of filter, the walls of the cavity, the long cylinders, the buckets under the cylinders and the disk-shaped objects nea

9、r the first and last cylinder are all made of metal. The long cylinders are connected to the top of the cavity; the buckets are connected to the bottom of the cavity. Cylinders and buckets dont touch. The disk-shaped objects near the first and last resonator are connected to the input and output tra

10、nsmission lines and provide the necessary coupling to the source and the load. We will call these objects antennas in this document. They are near the first and last cylinders, but never touch them. Each cylinder-bucket combination is a resonating structure. At this stage, without restricting oursel

11、ves, we can choose many dimensions in the filter relatively freely. We make the following choices:Cavity dimensions 280 x 30 x 120 mmResonator diameter 10 mmBuckets inner diameter 12 mmBuckets outer diameter 16 mmBuckets height 15 mmAntennas diameter 26 mmAntennas thickness 6 mmSix dimensions remain

12、, and these six will be crucial in obtaining the desired filter characteristic: The length of the first and last resonating cylinder (both have equal length) The length of the five interior cylinders (all five have equal length) The distance between an antenna and its nearest cylinder Three distance

13、s between neighboring cylinders (remember the filter is symmetric) With traditional filter design methods, obtaining the correct dimensions is a time-consuming task that commonly takes several weeks. Filter design with a circuit simulator, on the other hand, is relatively straightforward. Filter the

14、ory provides the values for the lumped inductors and capacitors that are needed to obtain the desired filter characteristic. First, we will show how to design a circuit that not only has the desired filter characteristic, but also lends itself to implementation with microwave components. In such a c

15、ircuit, we use series L and C for each resonator, i.e. the cylinder-and-bucket combinations, and impedance invertors to represent the distances between adjacent resonators. Second, we will show how one can determine relationships between components in the circuit and dimensions in the physical filte

16、r. Third, we will present an iterative procedure between the electromagnetic field solver and the circuit simulator to optimize the design. The procedure converges very quickly.2 Circuit Representation of the FilterIn order to design an order-seven band-pass filter around 400 MHz with a 0.1 dB rippl

17、e, filter theory tells us to start with an order-seven low-pass filter, normalized to 1 radian/s. The normalized filter is to have a 0.1 dB ripple, like the desired band pass filter. The source and load impedances of the normalized low pass filter are normalized to 1 Ohm. This circuit is shown in Fi

18、g. 3 and its characteristic in Fig. 4. Filter theory provides us with the values for the inductors and the capacitors, denoted by g1 through g7 in the figure. These values are in our case g1=g7=1.1812 Hg3=g5=2.0967 Hg2=g6=1.4228 Fg4=1.5734 F.Fig. 3 Normalized low-pass filter circuit, starting point

19、for design procedureFig. 4 Filter characteristic for the normalized low-pass filter in Fig. 3 The step-by-step procedure from this normalized low-pass filter circuit to the final band-pass filter circuit is presented in detail in Appendix A. Here, we show an outline of the major steps.An important s

20、tep is the replacement of shunt capacitors by series inductors and impedance inverters. Basically, an impedance inverter transforms impedances in the same way as a quarter-wave-length transmission line, but independent of frequency. The resulting circuit is shown in Fig. 5. This is still a normalize

21、d low-pass filter with the same characteristic as the circuit in Fig. 3. The reason for this change is that at microwave frequencies it is often impossible to realize the ladder circuit consisting of series inductors and shunt capacitors. Depending on the basic structure either series elements or sh

22、unt elements are easily realizable but often not both in the same structure. Taking advantage of impedance inverters, it is possible to transform shunt capacitors into series inductors. In the physical filter these impedance inverters will be realized by couplings between the coaxial resonators.Fig.

23、 5 Normalized low-pass filter without shunt capacitorsFollowing a standard procedure, we take the following steps to derive the desired band-pass filter model:(1) De-normalize the low-pass cut-off angular frequency from 1 rad/s to bw rad/s.(2) Transform the low-pass filter to a band-pass filter with

24、 a relative bandwidth of bw and a center angular frequency of 1 rad/s by inserting a 1 F capacitor in series with every 1 H inductor.(3) De-normalize the center frequency to 400 MHz by choosing L=1/(24E8) H and C = 1/(24E8) F. (4) De-normalize the port impedances from 1 Ohm to the usual 50 Ohm by in

25、troducing impedance inverters at the input and output with coupling coefficients of 50.(5) Introduce finite quality factors to the individual resonators by adding a series resistor to each resonator.(6) Introduce individual resonant frequencies to the first and last resonators to be able to be able

26、to take the frequency shift due to the coupling antennas into account.(7) Add a homogeneous transmission line of length ZUL between filter input/output and port 1 / port 2 to be able to adjust the phase due to the connectors.This gives us the filter shown in Fig. 6. The procedure outlined above is p

27、resented in more detail in Appendix A.Fig. 6 Final filter circuit, representing the desired band pass filterIn this circuit, every LC pair resonates at 400 MHz. Further K12, K23, K34 and QL have been defined as (1) and (2)where bw is the relative bandwidth and gi is the ith g value from filter theor

28、y.Notice that, since the g values are known from filter theory, we still know the values of the all the components in the circuit, even through the components have changed considerably in the process.Filter theory 2 tells us that Ki,i+1 and QL have important physical meanings. Ki,i+1 is known as the

29、 coupling constant between adjacent resonators. If we have just two resonators in the cavity, with a very light coupling to the source and the load, then the relation between coupling constant K12 and resonant frequencies f1 and f2 is given by K12 = 2(f2-f1) / (f2+f1) . (3)QL is known as the loaded

30、Q of the circuit. If we have just one resonator in the cavity, coupled to source and load, the relation between QL , resonant frequency fR and 3-dB band width BW3dB is given by QL = fR / BW3dB (4)In the next section, we will link the components of this circuit to dimensions in the physical geometry

31、of the filter.3 Relationships Between Circuit Components and Physical Dimensions in the Microwave FilterAs explained in the previous section, every LC pair resonates at 400 MHz. In the microwave filter, we must choose the length of each resonator such that it resonates at 400 MHz. That will determin

32、e the length of each of them. Further, Ki,i+1 (i=1,2,3) are the coupling coefficients between adjacent resonators. Therefore, these three coefficients are related to the distances between adjacent resonators. Finally, QL is the loaded Q of the circuit. Therefore, in an otherwise lossless circuit, it is directly related to the distance between the fi