Design of Parallel Coupled Microstrip Bandpass Filter for FM Wireless Applications

Transcription

1 Desin of Parallel Coupled Microstrip Bandpass Filter for FM Wireless Applications Salima Sehier,Nasreddine Benahmed and Fethi Tarik Bendimerad Department of telecommunications University Abou Bekr Belkaid-Tlemcen P.O.Box 9, (3) Tlemcen, Aleria Nadia Benabdallah Department of Physics Preparatory School of Sciences and Technoloy (EPST-Tlemcen) Abstract Desin, analysis and optimization of a parallelcoupled microstrip bandpass filter for FM Wireless applications is presented in this paper. The filter is desined and optimized at a center frequency of 6 GHz. Half wavelenth lon resonators and admittance inverters are used to desin the filter. A brief description of coupled microstrip lines and immittance inverters is also included. Desin equations to compute physical dimensions of the filter are iven in the paper. The filter is simulated usin ADS (Advanced Desin System) desin software and implemented on Roer 43C substrate. and odd- mode characteristic impedances of parallel-coupled half-wave resonators are computed usin admittance inverters. These even and odd mode impedances are then used to compute physical dimensions of the filter [3]. The filter is implemented on Roer 43C substrate with dielectric constant of 3.38, loss tanent of. and substrate heiht of.58 mm. Keywords- Bandpass filter; coupled microstip lines; immittance inverter; Method of moments. I. INTRODUCTION Parallel coupled transmission-line filter in microstrip and stripline technoloy are very common for implementation of bandpass and band-stop filters with required bandwidth up to a % of central frequency. Due to their relatively weak couplin, this type of filter has narrow fractional bandwidth but instead has desired advantaes such as low-cost fabrication, easy interation and simple desinin procedure (desinin equations for the coupled line parameters such as space-ap between lines and line widths and lenths, can be found in classical microwave books []. This way, followin a well-defined systematic procedure, the required microstripfilter parameters can be easily derived for Butterworth and Chebyshev prototypes []. This paper presents the desin of a parallel-coupled microstrip bandpass filter centered at 6 GHz, bandwidth BW of MHz with minimum attenuation of -5 db at 6. GHz, pass-band ripple of.5 db. This frequency band is used by FM Wireless communication. The filter is desined usin half wave lon resonators and admittance inverters. Theory of eneral immittance inverters and coupled lines is briefly described. General layout of a parallel coupled microstrip bandpass filter is shown in Fi.. The filter structure consists of open circuited coupled microstrip lines. These coupled lines are quarter wavelenth, ( / 4 lon) and are equivalent to shunt resonant circuits. The couplin aps correspond to the admittance inverters in the low-pass prototype circuit. Even Y Z / Fiure. General layout of parallel coupled microstrip bandpass filter. II. IMMITTANCE INVERTER Immittance inverters play a very important role in filter desin. They are used to transform a filter circuit into an equivalent form that can be easily implemented usin various microwave structures. Immittance inverters are either impedance or admittance inverters. Because of the invertin action, a series inductance with an inverter on each side looks like a shunt capacitance and a shunt capacitance with an inverter on each side looks like a series inductance. Immittance inverters are shown in Fi.. Fiure. Immittance inverter.

2 Makin use of the properties of immittance inverters, bandpass filters may be realized by series (L-C) resonant circuits separated by impedance inverters (K) or shunt (L-C) parallel resonant circuits separated by admittance inverters (). To desin a bandpass filter, first of all a low-pass prototype circuit is modified to include immittance inverters. These low pass structures are then converted to bandpass circuits by applyin conventional low-pass to bandpass transformation [3]. (i ) 4sin sin n i i ( i sin For i =, 3 n. for n odd n coth for n even 4 (i 3) n ) n (5) (6) III. PARALLEL- COUPLED FILTER DESIGN The first step in desinin a filter is to determine the order of the filter, n required. The order of the filter can be determined from (), () and (3) [4] and [5]. From the result of the (3), the order of the filter can be determined from Fi.3. Fractional bandwidth, FBW () Bandpass transformation, ' FBW () Where: Ripple ln coth 7.37 sinh n The element values obtained are = 4 =, = 3 =.5963, =.967.The low- pass prototype elements values obtained can be represented as shown in Fi.4. Normalized frequency, (3) (8) ' The low-pass prototype elements values obtained can be Fiure 4. Low-pass filter prototype. The low-pass filter consists of series and parallel branch, - inverter is used to convert low-pass filter to bandpass filter with only shunt branch as shown in Fi.5. (7), (8) and (9) are used to obtain the -inverters [7]-[]. Fiure 3. Attenuation versus normalized frequency for.5 db ripple low-pass filter prototype. Based on the desin specification, a 3 rd order filter is required for FM Wireless application. The element values for 3 rd order usin.5 db equal ripple low-pass prototype are determined from (4), (5) and (6) where =. [6]. i sin n (4) Y FBW (7) j FBW j to n (8) Y Fiure 5. Bandpass filter prototype j j n, n FBW (9) Y n n, n

3 db(s(,)) db(increase_len..s(,)) db(reduced_len..s(,)) db(s(,)) db(s(,)) Where FBW is the fractional bandwidth of the bandpass filter, j+ are the characteristic admittances of the inverters and Y is the characteristic admittance of the terminatin lines. To realise the -inverters, even- and odd-mode characteristic impedances of coupled lines, are determined by usin relations () and (). The calculated results are listed in Table I. This type of filter was studied by [8] at a center frequency of.49 GHz for Satellite Receiver. The result of the microstrip filter simulation is shown in Fi.7. In the plot, we can find out that the center frequency of the filter has deviated from the specified frequency 6 GHz. So the desin can t be used in practice. Z Y Y e j j Y j () Z Y Y o j j Y j () Where j = to n TABLE I. PARAMETERS OF PARALLEL- COUPLED FILTER Fiure 7. Scatterin parameters of the desined filter usin conventional method Order of the filter n admittance inverters j+/y Even-mode impedance Z e Odd-mode impedance Z o Based on the -even and odd characteristic impedance shown in Table I, the dimensions of the coupled lines can be obtained usin ADS Line Calculator, LineCalc. The lenth of coupled line is quarter wavelenth, / 4 lon. The physical dimension of the coupled lines, which is computed by LineCalc is shown in Table II. Fi.8 shows the effect of chanin coupled line lenth on the filter response. In this experiment, the width and space are kept constant while the lenth is simulated for both % increment and % reduction. As the lenth reduces, the frequency band shifted to the riht. Based on (), as uided wave lenth, ets shorter, the center frequency, f will increase and shifted towards hiher frequency band. Attenuation and insertion losses are not affected by the chane of lenth % -% TABLE II. PHYSICAL DIMENSSION OF COUPLED LINE Line description Width Lenth Gap (mm) (mm) (mm) 5 ohm-line Coupled lines and Coupled and lines 4 and A 3 rd order parallel-coupled bandpass filter as shown in Fi.6 is setup usin ADS to study the effect of optimizin the different parameters of coupled line. The circuit dimensions are based on Table II Fiure 8. 3 ( mm) () f ( GHz) Effect of chanin coupled line lenth Fi.9 shows the effect of chanin coupled line width on the filter response. In this experiment, the lenth and space are kept constant while the width is simulated for both % increment and % reduction. As the width increases, insertion loss increases. eff Fiure 6. Circuit of investiatin coupled line parameters.

4 db(s(,)) db(s(,)) db(s(,)) db(increase_space..s(,)) db(reduce_space..s(,)) db(s(,)) db(s(,)) db(s(,)) db(increase_width..s(,)) db(reduced_width..s(,)) Fi. shows the effect of chanin coupled lines pace on the filter response. In this experiment, the width and lenth are kept constant while the space is simulated for both % increment and % reduction. As the space increases, insertion loss increases as the couplin between the lines become weaker. IV. +% +% Fiure 9. Effect of chanin coupled line width % Fiure. Effect of freq, chanin GHz coupled line space OPTIMAL DESIGN OF THE COUPLED MICROSTRIP FILTER To reach the specified request, we adopt the optimal desin ability ownin by ADS, numerical software based on - the use of method of moments (MoM) [8] and [] (Fi.). We put the - OPTIM controller in the schematic as well as three GOAL controllers to specify the optimal oals which we want -3 to et after optimization. The paper sets three GOAL controllers -4which define the band-pass insert loss, alias attenuation and reflect ratio respectively. To make the insert -5 loss in band as least as possible, a 3 db loss is set when the frequency is between 5.9 and 6. GHz. -% We use VAR components to set the tunable parameters such as microstrip width, ap and lenth. In this desin, the filter is symmetrical eometrically, indeed only two roup different values are needed to determined durin the optimization which are set to be (w,s, l) and (w,s, l). The parameters in the VAR component should set around the values which we have calculated with conventional method. As the coupled microstrip filter is sensitive to the increment and decrease of its dimension, the value should not deflect the centre too much to lessen the time exhausted in the optimization. The plot of the amplitude versus frequency after optimization is shown in Fi.. In the plot, we can see clearly that the centre frequency of the filter has been adjusted to 6 GHz and the correspondin insert loss is less than db, the reflect ratio in pass band is -9.36dB with a dB attenuation in the alias frequency, indicatin that the request performance is well satisfied m freq= 6.34GHz db(s(,))=-9.36 m3 m freq= 6.GHz db(s(,))=-.94 m m m3 freq= 5.64GHz db(s(,))= Fiure. Scatterin parameters of the desined and optimized bandpass filter. After the optimization, we update the dimentions of the microstrip with the new parameters which are derived from the optimization. The final refined parameters of the dimension are listed in Table III. TABLE III. PHYSICAL DIMENSSION OF COUPLED LINE Line description Width Lenth Gap (mm) (mm) (mm) 5 ohm-line Coupled lines and Coupled lines and V. CONCLUSION In this paper the detail procedures for desinin a parallelcoupled bandpass filter for FM Wireless applications, has been presented. For the selected center frequency of 6 GHz and on a substrate with a dielectric constant of 3.38, our filter is desined and optimized. Half wavelenth lon resonators and admittance inverters are used to desin the bandpass filter. The optimization function ownin by ADS software is an Fiure. Layout of parallel-coupled microstrip bandpass filter

5 efficient tool to amend the drawback of conventional method with theoretic formulas. REFERENCES [] H. Karimi zarajabad and S. Nikmehr, A Novel Fractal Geometry for Harmonic Suppression in Parallel Coupled- Line Microstrip BandPass Filter, IEEE 8. [] Miuel Bacaicoa, David Benito, Maria. Garde, Mario Sorolla and Marco Gulielmi, New Microstrip Wily-Line Filters with Spurious Pass-band Suppression, IEEE Transactions on microwave theory and techniques, vol. 49, NO. 9, September. [3] Rashid Ahmad Bhatti, ahanir Khan Kayani, Desin and analysis of parallel coupled microstrip band pass filter, nd International Bhurban Conference on Applied Sciences and Technoloy,Bhurban,Pakisten. une 6-, 3. [4] G. Mattaei, L. Youn, and E.M.T. ones, Microwave Filters, Impédance Matchin Networks, and Couplin Structures, Artech House, Norwood, MA, 98. [5] Selinda Lim, Huey Pin, Desin and Modification of Parallel-coupled Bandpass filter, School of Electrical Enineerin and Computer Science, University of Newcastle, Callahan, NSW 3 8, Australia. [6] Hon,.S., M., Microstrip Filter for RF/Microwave Applications, A Wiley- Interscience Publication, Canada,. [7] D. M. Pozar, Microwave Enineerin, ohn Wiley & Sons Inc., 998. [8] Yanlin Hao, Binfa Zu, and Pin Huan, An optimal microstrip filter desin method based on Advanced Desin system for satellite receiver, Proceedins of IEEE International Conference on Mechatronics and Automation, 8. [9] Bey-Lin Su, Ray Yueh-Min Huan, 5.8GHz Bandpass Filter Desin Usin Planar Couple Microstrip Lines, IEEE, 4. [] Annapurna Das and Sisir K Das, Microwave Enineerin, MacGraw Hill, p35, [] en-tsai Kuo, Senior, Meshon ian, and Hsien-en Chan, Desin of parallel- Coupled Microstrip Filters With Sppression of Spurious Resonances Usin Substrate Suspension, IEEE Transactions on microwave theory and techniques, vol. 5, NO.,anuary 4 [] Tunin, Optimization and Statistical Desin, Ailent Technoloies, May 3.