HIGH PERFORMANCE MICROSTRIP BANDPASS FILTERS WITHNONSYMMETRICAL PARALLEL COUPLED LINES

M. J. Rosario*, J. Costa Freire', and R. Sorrentino+

ABSTRACT

A new improved technique for the design of bandpass microstrip filters based onthe image parameter concept is presented. Realizability is guaranteed bymanufacturing constraints being incorporated into the design procedure.Passband limitations associated with a previous technique are circumvented bythe use of nonsymmetrical parallel-coupled sections. Both computed andexperimental results demonstrate excellent filter performance in terms of bandpassattenuation, associated with a reduced number of elements.

INTRODUCTION

Bandpass filters in printed circuit configuration are usually realized in the form ofparallel coupled resonators, that are designed according to well establishedprocedures [1], [2]. In the practical implementation of these filters however,independently of the dielectric substrate adopted, unsatisfactory results are oftenobtained due to a number of technological aspects (inclusion of redundanttransmission line elements, impractical impedance levels, impractical couplinglevels, etc.). Some of these problems were removed by the design methodpresented in [3]. The direct application of the image parameter concept todistributed structures eliminates any transformations from lumped to distributedprototype and makes it possible to incorporate manufacturing constraints into thedesign procedure [4, 5].

The main limitation of the design procedure proposed in [3] is due to the lowimpedance level of the coupled line cells. In order to match the filter to the externalimpedance level Zo (usually 50 ohm) additional matching sections wereintroduced. Although filters with wide passbands were obtained, the passbandinsertion loss was still relatively high, generally around 0.2-0.5 dB.

Centro de Electr6nica Aplicada U.T.L. (I.N.I.C.) and Instituto Superior T6cnicoAv. Rovisco Pais, 1096 Lisboa, Portugal, Ph. 351-1-800637 Telex 63423 ISUTL P Fax 351-1 -899242

+ University of Rome "Tor Vergata" Via O. Raimondo, 00173 Rome, Italy Ph. 39-6-24990416 Fax 39-6-2490519

1085

A generalization of the technique presented in [3], which overcomes the abovementioned limitations is presented in this paper. Nonsymmetrical coupled linesections are introduced as constituting elements of the filter. A nonsymmetricalcoupled line, in fact, makes it possible to combine in the same element both thefiltering properties and the required impedance level transformation. Byeliminating the need for matching sections, passbands of the order of 40% withattenuations of the order of 0.05 dB or less are easily obtained.

FILTERS DESIGN

The filters result from the cascade of a number of cells consisting of symmetricaland/or nonsymmetrical coupled line sections. The automatic design procedure,implemented on a PC, is as follows:

1. From the passband specifications (frequency band and attenuation) asymmetrical coupled line cell is synthesized on an image basis [3], with the highestimage impedance level K allowed by the technology. For microstrip, K depends onthe smallest realizable w/h ratio, w being the strip width and h the substratethickness.

2. A nonsymmetrical coupled line cell is synthesized from the passbandspecifications and with the additional condition of providing the impedancetransformation from K to the prescribed level Zo. The image impedances, Z11 andZ12, of the 2 port nonsymmetrical lossless coupled line structure are obtained fromthe impedance-matrix description, in terms of the parameters of x and C modes [6]

_l__-z_l Z Z2X|CSC20_1 ZleR2 (1)Zii 2 cl 7-~~ZrRoII

where 0=21l/k is the electrical length of the cell. At the passband center frequency(0=K/2), the image impedance Z11 has the value

Zcl - Z,tlZfl = 2 =K (2)

In the passband, the image impedances are real. In accordance, the fractionalbandwidth B is given by

B3=2[1 - 2sin1(2_Q§Cj1)jn (3)Bc xu1 + ZCthZloctc

Bec-auseq of technnoloic-al fimitations, thA nonsyvmmAtrircal ceAllsq mav not besufficient to provide the necessary impedance transformation to match the filter to

1086

Zo in cases when Zo>>K. In such cases the impedance transformation can berealized in two steps by adding a quarter wave transformer at each end of the filter.

3. A number of cells are cascaded until the prescrbed attenuation in the stopbandis achieved. The stopband attenuation is calculated from the ABCD parameters ofthe total circuit. This leaves the passband behavior unaltered, as the cells areimage matched. Nonsymmetrical cells are used to provide the filter matching to theexternal impedance level.

4. The geometrical parameters of the coupled microstrip sections are obtainedfrom [7], [8]. Discontinuities are taken into account according to the formulasquoted in [8], [9].

5. The synthesized microstrip filter is analyzed by the computer program, thedispersion being included according to [10]. Discrepancies with the ideal responseare observed because of (i) different phase velocities of the even and odd modesin symmetrical sections, or the x and C modes in nonsymmetrical ones; (ii)discontinuity effects; (iii) imperfect matching at passband edges.

6. Coupled line lengths are adjusted by a simple optimization routine in order tocompensate the above unwanted phenomena and match the filter specifications.

THEORETICAL AND EXPERIMENTAL RESULTS

The synthesis method has been used to design filters with 10 to 40% bandwidthand passband attenuation down to 0.05 dB, under the following technologicalconstraints:

(wmin,Smin)>0.1mm; (Wmax, Smax)cl 0 (wmin, smin); 0.254mm<h<0.762mm;2.1 7<er<c1 0

In order to demonstrate the improvement obtained with the new design procedure,the comparisons with previous filter responses [3] are shown in Figs. 1 and 2. Allfilters are designed- assuming a RT/Duroid substrate 0.51 mm thick, with Fe,2.94.Filters of Fig. 1 have a moderate passband width from 4.6 to 5.5 GHz (17%). Thefilter of Fig. la consists of the cascade of 4 identical coupled line cells plus twoquarter wave sections, with a total length of 6X/4 at center frequency. The cellshave the following dimensions: w=.15mm, s=.44mm, 1=10.04mm, w being the stripwidth, s the separation, and I the length. The new designed filter of Fig. 1b, on thecontrary, consists of only 5 sections with no transformers, and has therefore anoverall length of 5X/4 at center frequency. The 3 central cells are identicalsymmetrical cells with w=.15mm, s=.44mm, and 1=10.04mm, while the two

nonsymmetrical cells have wi=.15mm, w2=.3mm, s=.2mm, and 1=9.91mm. In

1087

addition to the filter length reduction, the new filter has a passband attenuation of0.05dB compared to 1.38 dB of the previous filter.

4.6 5.5 F(GHZ) 7

Fig.1 - Comparson between symmetrical (a) and nonsymmetrical (b) parallelcoupled filters with 4.6 - 5.5 GHz passband

Fig 2 - Comparison between symmetrical (a) and nonsymmetrical (b) parallelcoupled filters with 4.6 - 6.6 GHz passband

Fig. 2 a,b shows the results for wide band (37%) filters. In this case the passbandattenuation is reduced only from 0.082 to 0.080 dB, but the filter length is reducedfrom 6 to 4X/4. The filter of Fig. 2b consists of only 4 cells, 2 symmetrical withw=.S15mm, s=.16mm, and 1=9.04mm, and 2 nonsymmetrical with wi=.15mm,w2=.16mm, s=.18mm, and 1=8.93mm.

In order to prove the validity of our analysis, the filters of Fig.1b and 2b were

simulated using a commercial package for linear microwave circuit analysisincluding losses and dispersion [11], and later fabricated and measured. Theresults are shown in Figs. 3 and 4 respectively. Both simulations and experimentsare in quite good agreement with our predictions.

1088

A

z 3009-

z 20w

25

20011

2150

gowI.-<-

at - Wtm - wn - 3:29:39 - UW

FIL

(a I I _/

DfrSll'FltL

-I ==-i- _ -

/ _In

-9 = , It A= =----'S 1.I / _3." s.m

r 2i.U s

.ee -;.

DBCESZIFIL

3B(S11)FIL(b'- t j _ _ " ~~~~~~~~~~~.

- t - -----"- -s-- --

- ItI_77__

WlUT IlI /IF1

I t \

11 I117-' N

I 1-11,I r

%

. Q--t-n ...iL _-. I'. f. 11

-s.m

9.5

Fig 3 - Simulated response of (a) the 5 section filter of Fig.lb(b) the 4 section filter of FIg.2b

CH1

c2

Fig. 4a - Expermental resj )onse of the filter of Fig. lblog MAC 5 / REF 0

I- -1/-1- -I- -, ,_ 1 -. I 8.55C.0Hz

START 3. 0000 GHz STOP 9. 0000 CHz

Fig. 4b - Expermental response of the filter of Fig. 2b

1089

$...,

-?-q -a I 7 - _ _ _ -I

r1 _ _ _T

Si I lo9 MAG S t/ REF O d.

- -= =-T --1.

START t 0000 GHs STOP 7. 0000 O;Hs

CH2

Cz

I I --r-

1- U I I

MMd IWAdWt. - lVtVU - 0:2*6.- FILM

(a)I Nk

1 r

CONCLUSIONS

A new effective design technique of microstrip bandpass filters has beenpresented. The realizability of the filters is guaranteed by the design procedurewhich incorporates the technological/manufacturng constraints. Filters with 10% to40% passband and low passband attenuation (down to 0.05dB) are easilyobtained with a reduced number of elements.

ACKNOWLEDGMENTS

This work was partially supported under CNR (Italy) - INIC (Portugal) agreement.

REFERENCES

[1] G.L. Mathaei, L. Young, and E.M.T. Jones, Microwave Filters, Impedance -Matching Networks and Coupling Structures, New York, McGraw-Hill, 1964.

[2] H. Howe, Stripline Circuit Design, Artech House, 1974.

[3] G. Bianchi, R. Sorrentino, M. Salerno, F. Alessandri, "Image ParameterDesign of Parallel Coupled Microstrip Filters", Proc. 18th EuropeanMicrowave Conf., 1988.

[4] M. Salerno, R. Sorrentino and F. Giannini, "Image Parameter Design ofNoncommensurate Distributed Structures: An Application to Microstrip Low-pass Filters", IEEE Trans. on Microwave Theory Tech., vol. MTT-34, pp. 58-65,Jan. 1986.

[5] G. Forte, M. Salerno, and R. Sorrentino, "The Planar-Circuit Image-ParameterMethod: A novel approach to the computer-aided design of MIC filters", AltaFrequenza, vol. 57, n.5, pp. 233-239, June 1988.

[6] V.K. Tripathi, "Asymmetrc Coupled Transmission Lines in an InhomogeneousMedium" IEEE Trans. on Microwave Theory Tech., vol. MTT-23, pp 734-7399Sept. 1975.

[7] M. Kirshning and R.H. Jansen, "Accurate Wide-Range Design Equations forthe Frequency-Dependent Characteristics of Parallel Coupled MicrostripLines", IEEE Trans. Microwave Theory Tech., vol. MTT-32, pp. 83-90, Jan.1984.

[8] S.S. Bedair, "Characteristics of Some Asymmetrical Coupled TransmissionLines", IEEE Trans. Microwave Theory Tech., vol. MTT-32, pp. 108-1 1 0, Jan.1984.

[9] T.C. Edwards, Foundations for Microstrip Circuit Design, John Wiley, 1981.

[10] V.K. Tripathi, "A Dispersion Model for Coupled Microstrips", IEEE Trans.Microwave Theory Tech., vol. MTT-34, pp. 66-71, Jan. 1986.

[1 1] "Touchstone 1.5 User Guide" - EESOF, Inc. March 1987.

1090