DESIGN OF BPF USING INTERDIGITAL BANDPASS FILTER ON CENTER FREQUENCY 3GHZ. 1 Anupma Gupta, 2 Vipin Gupta 1 Assistant Professor, AIMT/ECE Department, Gorgarh, Indri (Karnal), India Email: anupmagupta31@gmail.com 2 Assistant Professor & Head, Doon Valley Group of Institutes/ ECE & EEE Department, Karnal, India Email: er.vipingupta14@gmail.com Abstract In this paper a high performance Bandpass micro strip filter is presented. The filter is designed using interdigital bandpass filter to improve the performance of Bandpass filter. The designed filter is based on 5th order chebyshev low pass prototype with.1 db passband ripples. The filter operates with passband from 2GHz-4 GHz, produces a fractional bandwidth of 66%. The filter is designed on a FR4 substrate with relative dielectric constant of 4.4 and a thickness of 1.57mm. The simulated result using HFSS shows an insertion loss (S21) less than -1.8dB and return loss (S11) better than -8 db. Keywords- Microwave Filter, BPF, Microstrip, Interdigital Bandpass Filter, Quarter Wavelength. 1. INTRODUCTION The demand in high speed communication bandpass filter plays an important role. As after modulation all basic signals attain a band of frequency, therefore in order to separate one signal from other bandpass filter need to be employed at transmitting as well as at receiving end. Bandpass filters are highly applications specific. The frequency spectrum of Bandpass filter are from 2GHz to 4 GHz.A broadband filter using interdigital bandpass filter microstrip line resonator was presented in [1-3] with a different bandwidth. In BPF coupled microstrip transmission line are used where the two microstrip line of width W and separation S. these coupled line support two quasi-tem modes i.e. even and odd mode. These modes of propagation basically depend on the voltage polarities applied to the microstrip lines. When the two micros trip lines are applied with the same voltage polarities the structure is said to have even mode of excitation whereas if two microstrip lines are applied with the opposite voltage polarities the structure is said to have odd mode of excitation. These modes of excitation are shown in fig 1. Figure. 1: Field distribution for (a) even mode excitation (b) odd mode excitation. According to the assigned voltage polarities, there is a corresponding charge distribution on the microstrip lines which governs the total capacitance of the coupled lines. For even and odd mode, as voltage polarities are different therefore the associated capacitance is also different. Because of different capacitance associated with two modes the velocity of propagation and characteristics impedance (Z oo and Z oe ) for them is also different. These impedances are crucial factors for determining the physical dimension like width and spacing of the 135 P a g e IJRREST h t t p : / / i j r r e s t. o r g / i s s u e s /? p a g e _ i d = 1 2
coupled resonators. Detailed mathematical analysis is given in [4]. These coupled line structure can be used in following applications: (A) Filters, delay lines, and matching networks - often using arrays of parallel coupled microstrip as resonant elements. (B) Directional couplers - for use in a variety of circuits including balanced mixers, balanced amplifiers, phase shifters, attenuators, modulators, discriminators, and measurement bridges. Here we deal only with the first application as per our goal. The filter we discuss here are only interdigital filter. This configuration is commonly used for microstrip implementation. The basic structu re consists of quarter wavelength resonators at mid-band frequency with short circuit at one end and open circuit at the other end of alternate orientation. Fig 2 shows a tapped interdigital bandpass filter consisting of n quasi-tem mode resonators. The physical dimensions of the resonators are indicated by: lengths l 1,l 2 l n and the widths W 1, W 2 W n. Coupling is achieved by the fields fringing between adjacent resonators separated by S i,i+1. The filter input and output use the tapped lines with a characteristic admittancey t, which may be set to equal the source/load characteristic admittancey o. Electrical length θ t is always measured from the short circuited end. Y 1 = Y n denotes the single microstrip characteristics admittance of input/output resonators. 2. BPF FILTER DESIGN The design is based on a low pass chebychev filter prototype with (.1) db passband ripples. The equivalent structure for short circuited stub filter is shown in figure2 [5]. Figure. 2: Genral configuration of Interdigital Bandpass Filter Here, BPF with centre frequency 3GHz and fraction bandwidth 66% is designed on a 1.57 mm substrate height on FR4 substrate with dielectric constant ε r =4.4 and simulated using HFSS. 2.1 Design Equations for Interdigital Bandpass Filter [5] θ ( ) θ For = 1 to n-1 Where are characteristics admittance of J inverters. θ Where are Chebyshev lowpass prototype values. 136 P a g e IJRREST h t t p : / / i j r r e s t. o r g / i s s u e s /? p a g e _ i d = 1 2
For =2 to n-2 (1.1) For =2 to n-2 For calculating the length of resonator: ( ( ) ( ) ) ( ) The model in figure 2 is derived from J inverters by using conventional filter design and the line admittances, Yi,i+1 given to fulfill the specifications. The separation distance between the stubs is denoted by S whereas stub length is given by li. For designing the proposed filter first a 5th order chebychev low pass prototype with.1db passband ripples is selected[5]. Low pass filter prototype parameters are given as : g0=g6=1,g1=g5=1.1468, g2=g4=1.3712and g3=1.9750. Table1.Calculated values of even and odd mode impedance for resonator using equ.1.1 Characteristics impedance of input and output resonators Even mode characteristics Impedance Z oe1,2= Z oe4,5 O dd mode characteristics Impedance Z oo1,2= Z oo4,5 Even mode characteristics Impedance Z oe2,3= Z oe3,4 O dd mode characteristics Impedance Z oo2,3= Z oo3,4 0.02378 Ω 70.028 Ω 30.048 Ω 60.314 Ω 32.279 Ω Here using equ. 1.1 calculating impedance values and these values are used for calculating width and spacing of resonator on a given substrate; first we calculate coupling coefficient and then using online coupled line calculator these dimensions are adjusted in order to match the required coupling coefficient. For calculating length of the resonators, first even and odd mode effective dielectric impedance (ε re &ε ro ) are calculated usingmatlab code from appendix B and then use eq. (1.2). Alternatively, the online calculator gives this physical length corresponding to an electrical length of 90 deg. The length of input and output resonators are different from what is calculated because of tapping. 2.2 Calculated value of length and width for resonators and tapped line The tapped line is always taken to be of 50Ω and is at a length of θ t from the short circuited ending where θ t is given by: 137 P a g e IJRREST h t t p : / / i j r r e s t. o r g / i s s u e s /? p a g e _ i d = 1 2
( ) ( ) Its physical length l t is then calculated. Table 2: Calculated value of length and width for resonators and tapped line Sr.No 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Dimensions Structure Notation (mm) Width and length of 1 st 2.8 and 13.28 resonator is W 1 and L 1 Width and length of 2 nd 2.8 and 12.78 resonator is W 2 and L 2 Width and length of 3 rd 2.8 and 11.7 resonator is W 3 and L 3 Width and length of 4 th 2.8 and 12.78 resonator is W 4 and L 4 Width and length of 5 th 2.8 and 13.28 resonator is W 5 and L 6 Width (W t ) and length 2.7 and 4.6 (l t ) of tapped line 0.095 resonator 1 & 2 0.153 resonator 2 & 3 0.153 resonator 3 & 4 0.095 resonator 4 & 5 The actual dimensions for which the filter is designedare some what different from that calcuated using design equations and are given in Table 2. Figure. 3: Design of Filter Using HFSS 138 P a g e IJRREST h t t p : / / i j r r e s t. o r g / i s s u e s /? p a g e _ i d = 1 2
The strucutre dimension calculated using design equations for interdigital resonators filter provides a FBW of 60%. The actual bandwidth of filter resulting from the design equations are different from that used for design, this difference between the two is mainly due to large FBW. The dimensions are then optimized using HFSS simulator in order to get the desired FBW. Using the dmensions of Table 2 the structure is designed in HFSS 14 and the simulated results are shown in Fig 3.The designed structure is shown in figure 3. The structure in figure 2 is simulated using HFSS.with stub dimension shown in table 2. To get better results dimensions are adjusted slightly to get better results. 3. RESULTS AND DISCUSSION Filter is designed using a low cost 1.57 mm thick FR4 substrate of relative dielectric constant ε r =4.4. The simulated results of scattering parameters using HFSS14is shown below in figure 3. The measured results show an in-band insertion loss of -1.8dB in the entire passband from2ghz -4GHz thus providing low insertion loss when compared with insertion loss. The filter shows return loss better than -8dB thus good impedance matching is achieved at the I/O ports. The designed filter gives a 3-dB fractional bandwidth of 66%. 4. CONCLUS ION Figure. 4: Simulated Result (a) S11 (db) and S21 (db) (b) phase response The results shows that the structurehas a passband from 2 GHz 4 GHz with a maximum insertion loss of less than -1.8 db and Return loss better than -8 db. f The passband response, however repeats at threetimes of first resonating frequency because the quarter wavelength resonators are used.the interdigital topology is mainly used for large number of applications specifically having medium FBW like WLAN, Wi-MAX etc. 5. REFERENCES [1] Arne BrejningDalby, Interdigital Microstrip Circuit Parameters Using Empirical Formulas and Simplified Model, IEEE, Transactions on Microwave Theory and Techniques, Vol. no.-27, 8, august 1979. [2] M. Ramadan and Westgate, Impedance of Coupled Microstrip Transmission Lines, Microwave J., pp. 30-34, July 1971. [3] R. Levy, A New Class of Distributed Prototype Filters with Applications to Mixed Lumped/Distributed Component Design, IEEE, Transactions on Microwave Theory and Techniques, Vol. no.-18, December 1970,1064 1071. [4] T.C. Edwards, Foundation of Microstrip Circuit Design, Third Edition, John Wiley & Sons, 2000. [5] Jia Sheng Hong and M.J. Lancaster, Microstrip Filter for RF/Microwave Applications, John Wiley & Sons, 2001. [6] S. C. Saha and U. Hanke, Tunable Band-Pass Filter Using RF MEMS Capacitance and Transmission Line, Progress In Electromagnetic Research C, Vol. 23, 233-247, 2011 [7] Ralph Levy, fellow IEEE and Seymour B, COHN, fellow IEEE, A History of Microwave Filter Research, Design, and Development, IEEE, Transactions on Microwave Theory and Techniques, vol. No.-32, 9 September, 1984. [8] David M. Pozar, Microwave engineering, Third Edition, John Wiley & Sons, 2005 139 P a g e IJRREST h t t p : / / i j r r e s t. o r g / i s s u e s /? p a g e _ i d = 1 2