Compact microstrip coupled-line bandpass filter with two cross-couplings for creating multiple transmission zeros

Compact Microstrip Coupled-Line Bandpass Filter

With Two Cross-Couplings for Creating

Multiple Transmission Zeros

Chao-Huang Wu, Yo-Shen Lin, Chi-Hsueh Wang, and Chun Hsiung Chen

National Taiwan University, Taipei 106, Taiwan. (email: chchen@ew.ee.ntu.edu.tw)

Abstract — A compact coupled-line bandpass filter with

two capacitively cross-coupled paths to create multiple transmission zeros is proposed. The locations of transmission zeros can be adjusted by varying the values of cross-coupled capacitances so as to improve the filter selectivity. Specifically, a 4th-order microstrip coupled-line bandpass filter centered at 4.9 GHz with a 3-dB bandwidth of 6% and six transmission zeros is implemented and examined.

I. INTRODUCTION

In microwave communication system design, a filter with compact size, low insertion loss, and good selectivity is usually needed to reduce the cost and to enhance the system performance. In order to achieve better selectivity, many improvements on creating suitable transmission zeros were reported. In [1]-[2], the filters with a cross-coupling between resonators to generate transmission zeros were examined. Multiple transmission zeros were obtained by using suitable input, output, and interstage tapped-couplings [3]. In [4]-[5], quarter-wavelength (λ/4) open stubs and capactively coupled gaps were introduced in the conventional coupled-line filter structure to create the transmission zeros at stopband. The trisection and quadruplet microstrip bandpass filters [6]-[7] based on folded λ/4 resonators were proposed to achieve very compact circuit sizes with one or two transmission zeros. The compact hairpin filter with asymmetric tapping feed lines to produce transmission zeros was also discussed in [8]-[9]. In our previous works [10]-[12], by introducing a capacitive cross-coupling effect, two transmission zeros at upper and lower stopbands may be created for improving the filter selectivity.

In this study, the 4th-order microstrip bandpass filter discussed in [13] and shown in Fig. 1 is extended to develop a novel filter structure as shown in Fig. 2 which has two capacitive cross-couplings to create multiple transmission zeros. Comparing with the basic 4th-order bandpass filter in Fig. 1 which has a cross-coupled path provided by the capacitance C2, the proposed filter has an

additional cross-coupled capacitance C3 introduced

directly between the input and output ports as shown in Fig. 2. Therefore, the new filter structure may create multiple transmission zeros at the upper and lower stopbands such that the selectivity of the proposed filter may further be improved while keeping the same circuit size.

Fig. 1. Circuit model of the 4th-order bandpass filter in [13] with a capacitively cross-coupled path to create two transmission zeros.

Fig. 2. Circuit model of the proposed 4th-order bandpass filter with two capacitively cross-coupled paths to create multiple transmission zeros.

II. FILTERSTRUCTURE ANDIMPLEMENTATION Fig. 2 shows the proposed filter circuit model with two capacitively cross-coupled paths to create multiple transmission zeros. By neglecting the cross-coupling effects, i.e. C2 = C3 = 0, the coupled-line section with

open circuited terminal may be equivalent to a J inverter along with two transmission lines for narrow band [14] and the shunt capacitor C1 or inductor L with two

transmission line sections at its two ends may be equivalent to a K inverter [15]. Thus, the equivalent circuit model is the same as the one in [13].

Based on the circuit model in Fig. 2, a novel 4th-order microstrip coupled-line bandpass filter is implemented as in Fig. 3 which has two cross-coupled capacitors C2 and

C3 to generate four to six transmission zeros depending

on the values of capacitances C2and C3. Here, the

cross-coupled capacitor C2 is realized by the gap-coupled

configuration between the open-ends of two coupled-line sections. In order to achieve the desired amount of cross-coupling through the capacitor C3, the two coupled-lines

are bended by an angle θ1 and the transmission-line

Fig. 3. Layout of the proposed 4th-order microstrip coupled-line bandpass filter with two capacitive cross-couplings to create multiple transmission zeros. (W1=0.97mm, S1=0.5mm,

W2=1.17mm, S2=0.3mm, W3=1.53mm, S3=0.15mm, l1=7.08mm,

l2=1.4mm, l3=4.3mm, l4=4.2mm, l5=4.3mm, l6=0.8mm,θ1=70,

θ2=90, and d=1.1mm.)

Fig. 4. Measured and simulated results for the proposed 4th-order microstrip coupled-line bandpass filter shown in Fig. 3.

accomplished by the open stubs and the inductor L is realized by the via hole so as to fit the values of the K inverters.

The proposed filter structure is implemented using the microstrip configuration, and is fabricated on a Rogers RO4003c substrate (εr = 3.38, tanδ = 0.0023, and

thickness h= 0.508 mm).The simulated results by the full-wave simulator HFSS and the measured ones for the filter in Fig. 3 are shown in Fig. 4. The measured center frequency is at 4.9 GHz. The minimum measured insertion loss is 2.5 dB at 4.87 GHz, and the 3-dB bandwidth is 6%. Six transmission zeros at 2.38 GHz, 3.34 GHz, 4.33 GHz, 5.39 GHz, 5.89 GHz, and 6.48 GHz

Fig. 5. Full-wave simulated responses of the proposed 4th-order microstrip filter in Fig. 3 for (a) various values of S2to control

the values of capacitance C2, (b) various values of l6to control

the values of capacitance C3.

are observed as expected. Note that the inclusion of the cross-coupled capacitor C3 has created the multiple

transmission zeros to improve the filter selectivity but at the expense of degrading the stopband rejection level.

III. TRANSMISSIONZEROS

The locations of transmission zeros for the proposed filter may actually be adjusted by varying the parameter S2(Fig. 3) to control the value of C2and the parameters l6

and S3to control that of C3. Shown in Fig. 5 are the

full-wave simulated responses of the proposed filter with S2

and l6in Fig. 3 as parameters. As shown in Fig. 5(a), the

two left-most transmission zeros (with respect to the passband center frequency fo) will move and close to each

other and the transmission zeros at upper stopband will move toward the passband edge as the parameter S2 is

decreased to increase the value of C2. For the case S2 =

0.1 mm, only four transmission zeros are observed because some of transmission zeros would be too close to be distinguishable. As shown in Fig. 5(b), the inner pair of transmission zeros will move toward the passband edge and the stopband rejection level will degrade as the

Fig. 6. Full-wave simulated curves to relate the frequencies of transmission zeros to the parameters (a) S 2 (l6= 0.8mm, S3=

0.15mm), (b) S 3 (l6= 0.8mm, S 2=0.3mm), and (c) l 6 (S2=

0.3mm, S 3=0.15mm) in Fig. 3.

parameter l6is increased to increase the value of C3. For

the case l6= 0.2mm, the frequency response only has five

transmission zeros. Therefore, according to the movement tendency of transmission zeros in Fig. 5, the values of C2

and C3can be determined for the desired locations of the

transmission zeros so as to achieve the required fall-off rate at the passband edge and the desired level of

stopband rejection.

Shown in Fig. 6 are the detail full-wave simulated curves to relate the frequencies of the transmission zeros to the parameters S2, S3, and l 6. Comparing Fig. 6(b) with

Fig. 6(c), the parameter S3has more influence than l6on

the frequencies of transmission zeros. The parameter S3

can first be selected and the parameter l 6is then tuned to

fit the specification. Some transmission zeros would be too close to be distinguishable as the parameter S3is

increased or decreased. Note that the frequencies of transmission zeros might slightly be changed because of the dimensions variation in the fabrication process.

IV. CONCLUSION

In this work, a compact 4th-order microstrip bandpass filter with multiple transmission zeros has been proposed. By introducing two capacitively cross-coupled paths, the proposed filter exhibits multiple transmission zeros at upper and lower stopbands. The locations of these transmission zeros may simply be adjusted by varying the values of cross-coupled capacitances. The proposed bandpass filter is useful for application in the communication systems when better selectivity and good stopband rejection are required.

ACKNOWLEDGEMENT

This work was supported by the National Science Council of Taiwan under Grant NSC 93-2752-E-002-001-PAE and Grant NSC 93-2219-E-002-021.

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