440 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 13, NO. 10, OCTOBER 2003
Parallel-Coupled Microstrip Filters
With Over-Coupled End Stages for
Suppression of Spurious Responses
Jen-Tsai Kuo, Member, IEEE, Sin-Ping Chen, and Meshon Jiang
Abstract—In a parallel-coupled microstrip filter, end stages with over-coupling are designed to suppress the unwanted responses at twice the passband frequency (2 ). The inherent transmission zero of an over-coupled input/output stage is shown tunable. It is found that increasing the image impedance of the filter sections can further enhance the suppression. The designed bandpass filters thus have a wide upper stopband and improved passband response symmetry. Measured results of fabricated circuits show that the idea works very well.
Index Terms—Microstrip filter, spurious response.
I. INTRODUCTION
P
ARALLEL-COUPLED microstrip filters have been widely used in the RF front end of microwave and wireless communication systems for decades. Major advantages of this type of filter include an easy synthesis procedure [1], good repetition, and a wide range of filter fractional bandwidth [2].The traditional design of parallel-coupled microstrip filters suffers from the spurious response at twice the passband fre-quency ( ) [2], [3], which causes passband response to be asymmetric, reduces the width of the upper stopband, and could greatly limit their applications. It is resulted from the inequality of and , the even- and odd-mode phase constants, respec-tively, of the coupled lines for each stage. This problem becomes more severe if a dielectric substrate with relative high permit-tivity is used, since the two eigen-modes will exhibit a consid-erable difference in and .
Consequently, the ways to tackle this problem fall into two categories [4]: providing different lengths for the even- and odd-modes, and equalizing the modal phase velocities. In [3], [4], an over-coupled resonator is proposed to extend phase length for the odd-mode to compensate difference in the phase velocities. The structures in [5], [6] use capacitors to extend the traveling path of the odd-mode. The corrugated coupled microstrips in [7] are designed for equalization of the modal phase velocities. The stepped impedance resonators (SIRs) in [8], the method in
Manuscript received February 18, 2003; revised June 7, 2003. This work was supported in part by the National Science Council, Taiwan, R.O.C., under Grants NSC 91-2213-E-009-126, and in part by the joint program of the Ministry of Education and the National Science Council under Contract: 89-E-F-A06-2-4. The review of this letter was arranged by Associate Editor Dr. Shigeo Kawasaki. The authors are with the Department of Communication Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C. (e-mail: jtkuo@cc.nctu.edu.tw).
Digital Object Identifier 10.1109/LMWC.2003.818531
Fig. 1. Dependence ofjS j responses on R = (" =" ) for a coupled microstrip stage.
[9] and the wiggly-line coupled stage design in [10] are also effective in improving the rejection characteristics of the filter at .
In this letter, two techniques are incorporated into the design of parallel-coupled microstrip filters for suppressing the spurious response at . First, the over-coupling in [4] is used. It is found in this work that applying over-coupling to the end stages, rather than to every coupled stage, is sufficient to improve the filter characteristics in the upper stopband. Then the image impedance is increased to reduce the difference in and of each coupled stage, so that the suppression of the spurious harmonic can be enhanced. Sections II and III will explain the technical background of these two ideas, and compare the predicted and measured responses. Section IV draws the conclusion.
II. TRANSMISSIONZERO OF ANOVER-COUPLEDSTAGE
For a microstrip coupled stage, Fig. 1 shows the dependence of responses on , the ratio of the effective permittivity of the even-mode to that of the odd-mode. The re-sults can be easily obtained by deriving the -parameters of the two-port network, followed by converting to the -parame-ters [1]. For the ideal case with , the response has an inherent transmission zero at . When is increased, the zero moves to higher frequencies, and vice versa. For practical
KUO et al.: PARALLEL-COUPLED MICROSTRIP FILTERS WITH OVER-COUPLED END STAGES 441
Fig. 2. Layout for a third-order filter. Over-couplings are applied to the end stages.
coupled microstrips, is greater than unity. The passband re-sponses in the neighborhood of , however, do not change sig-nificantly when the value of is changed, since the derivative of the coupling response is zero at . Therefore when a stage is over-coupled, i.e. the coupling length is longer than , the passband response will be almost unchanged, and the in-herent zero will move to a frequency lower than . In other words, the effectiveness of increasing the coupling length of a coupled-line stage on the zero is equivalent to that of decreasing , which is also equivalent to increasing or decreasing . It implies that from the circuit design point of view, the inherent zero can be tunable, within a certain range, by merely adjusting the length for the over-coupling. Since the position of the zero is close to the band of the spurious responses, it can be used to enhance the filter performance around .
Fig. 2 shows the layout of a third-order Chebyshev bandpass filter, of which both the end stages are over-coupled. Fig. 3 plots the simulation and measured results for a third- and a fifth-order filters. The detailed passband responses are also plotted in the zoomed windows. The filters are designed by the classical syn-thesis method [1], followed with adding an extra section to the end stages for over-coupling. The lengths of the extra sections are chosen so that the peak of the spurious response is mini-mized. The IE3D [11] is used for the electromagnetic simula-tions. The simulation results for filters directly synthesized by the traditional method, i.e. without any over-coupling, are also plotted for comparison. The measured peak rejection levels for the filters with proper over-couplings are no more than 30 dB. It can be seen that a suppression of at least 20 dB to the spurious responses is obtained by using the over-coupled stages.
It is noted that the design in Fig. 2 is much simpler than that in [4], where over-coupling is applied to every coupled stage. The tuning in our design involves only two over-couplings at the end stages, while that in [4] involves the coupling coefficients of all the coupled stages, which include three-line sections or angled nonuniform coupled lines. The substrates in both works have a similar dielectric constant, but the thickness of our substrate is more than three times that of the counterpart. Nevertheless, the measured suppression to the spurious passband in Fig. 3(a) and Fig. 3(b) shows a similar level to that of the optimized filter in [4].
III. ENHANCING THE SUPPRESSION BY
INCREASING THEIMAGEIMPEDANCE
Suppressing spurious harmonics for parallel-coupled mi-crostrip filters on a substrate with larger is more difficult than those on a substrate with smaller , since the eigen-modes will exhibit more deviation in phase velocities, which is the very
Fig. 3. Simulated and measured responses for Chebyshev bandpass filters. The center frequencyf = 2 GHz and passband ripple = 0:1 dB. The substrate has
" = 10:2 and thickness h = 1:27 mm. (a) Responses of a third-order filter
with fractional bandwidth1 = 20%. The electrical lengths for over-coupling at the end stages are = 12:7 and = 17:2 . (b) Responses of a fifth-order filter with1 = 15%, = 15:1 and = 18:8 .
Fig. 4. Comparison of" and " for two pairs of coupled lines. Z = 50 :
W = 0:590 mm, S = 0:220 mm; Z = 80 : W = 0:206 mm, S = 0:396 mm.
442 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 13, NO. 10, OCTOBER 2003
Fig. 5. Simulated and measured responses for filters with image impedance
Z = 80 . The design specifications of the filter and the substrate are identical
to those in Fig. 3. (a) Responses of a third-order filter with1 = 20%, =
12:5 and = 12:9 . (b) Responses of a fifth-order filter with 1 = 15%, = 14:1 and = 15:6 .
reason that the spurious response arises. Thus, any technique for reducing the deviation between the modal phase velocities will be helpful in the suppression of unwanted harmonics. It is found that if the image impedance of a coupled stage can be increased, the difference between and of the coupled lines can be reduced at the same time. For keeping both system reference impedance and the passband response unaltered at the same time, the method in [12] can be invoked to design parallel-coupled filters with an image impedance other than 50 . In this letter, the image impedance of the filter is changed from 50 to 80 . Fig. 4 compares and of two
pairs of coupled microstrips, of which the dimensions are those for the first stages of the filters shown in Fig. 3(a) and Fig. 5(a). The relative deviation between and for the 50 case is reduced from 21.9% to 10.6% for the 80 case. The responses for two filters with are shown in Fig. 5(a) and Fig. 5(b), of which the specifications are identical to those of Fig. 3(a) and Fig. 3(b), respectively. The over-coupling is also included in the circuit design. As indicated for both filters, the measured attenuation levels at are better than 50 dB. For the particular case studies shown, the suppression is enhanced by 15 dB by increasing the image impedance of the filter. It is to be noted that the passband responses are close to being unchanged.
IV. CONCLUSION
This letter shows an effective example of common par-allel-coupled microstrip filter design. Suppression of the spurious response is achieved by introducing over-coupling to the end stages and increasing the image impedance of the filter. The over-coupling is applied to the end stages only. A coupled microstrip stage with higher image impedance is shown to have smaller difference in and , and parallel-coupled microstrip filters with higher image impedances also show an improved rejection at .
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