Huan-Huan Xie, Yong-Chang Jiao, Bing Wang, and Fu-Shun Zhang

Bandpass filters are key building blocks in modern communications systems. Ideally, they channel desired signals with low loss and reject unwanted signals and noise from a communications receiver. Achieving good electrical performance with a microwave bandpass filter involves a number of tradeoffs, but a design based on meandered parallel coupled lines shows great potential to achieve outstanding performance in a compact size. To suppress the filter's harmonic passband and achieve a wider stopband bandwidth, two nonuniform defected microstrip structure (DMS) circuits were added on the two ports of the filter, and the difference in performance with the DMS units was clear. Without the DMS units, the S21 forward transmission level is -11.4 dB within the harmonic passband at 4.93 GHz. When the DMS units are added, the harmonic passband is suppressed and the S21 transmission level is reduced to -40.7 dB. simulations using commercial method-of-moments (MoM) software were performed, with good agreement to measurement results for a fabricated prototype filter.

Microstrip is a convenient and versatile transmission-line technology for fabricating compact bandpass filters for wireless communications applications. These filters must be small in size, but with low passband loss and high stopband rejection. as one of the most commonly used forms of bandpass filter, parallel-coupled microstrip bandpass filters1 have attracted much attention for a wide range of applications. Various modified parallel-coupled-line microstrip bandpass filter configurations have been proposed to reduce the size of these filters.2-5

In ref. 2, for example, capacitance-loaded parallel coupled lines were used to reduce the size of the bandpass filters. In ref. 3, parallel coupled lines with branch lines were successfully implemented for reducing the length of a bandpass filter. In ref. 4, corrugated coupled lines helped miniaturize bandpass filters, and in ref. 5, meandered parallel coupled-lines helped reduce the size of bandpass filters reported in that work. These filters encounter problems with spurious harmonic passbands, however, which appear near the upper end of the filter passband. The result of the harmonic passbands is narrow stopband.

Several methods have been adopted to suppress harmonic passbands in compact bandpass filters.6,7 Defected ground structure (DGS) constructions have been used in microstrip bandpass filters with some success.6 But DGS approaches involve etching slots on the ground plane, which inevitably causes energy leakage from the ground plane. In ref. 7, C-shaped electronic-band-gap (EBG) structures were implemented to control the harmonic passband. Unfortunately, a disadvantage of this approach is the need to use three or more layers of PCB substrate to fabricate the EBG structures.

A more practical approach for achieving effective suppression of harmonic passbands in a miniature bandpass filter design was through the use of DMS and meandered parallel coupled lines. Using DMS can eliminate the disadvantages of the DGS and EBG approaches. An experimental bandpass filter with DMS suppressed the harmonic passband at 4.93 GHz and achieved a -30-dB stopband bandwidth from 3.4 to 6.50 GHz. Filter designs were modeled by means of commercial simulation software and were fabricated and measured to validate the theoretical designs. The measured results agreed closely with the theoretical data.

Figure 1 shows the physical configuration of the proposed bandpass filter. It is composed of meandered parallel coupled lines. The coupled lines are somewhat more compact than traditional parallel coupled lines.1 The coupled lines in the proposed filter are connected to two 50-Ω microstrip line ports, each with a width of 2.8 mm. Bandpass filters based on parallel coupled lines usually have harmonic passbands, and that is no different when the parallel coupled lines are meandered to save size. To suppress the signal levels of the harmonics passbands, two nonuniform DMS are loaded on the microstrip line ports. According to the layout in Fig. 1, the dimensions for the bandpass filter are set as follows m = 62.4 mm, n = 20 mm, c = 18.5 mm, c1 = 7.4 mm, c2 = 8 mm, b1 = 7.26 mm, b2 = 2.9 mm, b3 = 0.9 mm, d1 = 6.8 mm, d2 = 6 mm, g1 = 0.2 mm, g2 = 0.5 mm, w = 2.8 mm, and w1 = 0.5 mm. The bandpass filter was fabricated on a 1-mm-thick FR-4 printed-circuit-board (PCB) substrate with relative dielectric permittivity of 2.65.

All simulations were performed by means of Version 12 of the method-of moments (MOM) electromagnetic (EM) simulator IE3D from Mentor Graphics (formerly Zeland Software). Bandpass filters with and without DMS were simulated, and the simulated S-parameters are shown in Fig. 2. According to the data curves from Fig. 2, the reflection coefficient (S11) of the bandpass filter without DMS has a -10 dB bandwidth from 2.11 to 2.47 GHz, with less than 0.9 dB passband insertion loss from 2.11 to 2.47 GHz. There is a harmonic passband for this filter at 4.76 GHz. The S21 level within the harmonic passband is -10.6 dB. After adding the DMS, the filter's harmonic bandpass is suppressed and the S21 level at 4.76 GHz is reduced to -36.8 dB. The -30 dB stopband bandwidth of the bandpass filter with the DMS is 3.18 to 6.50 GHz.

The DMS units play a key role in suppressing the harmonic passband of the bandpass filter. It is necessary to analyze the effect of the DMS units on the harmonic passband in detail. A transmission line model for the DMS is shown in Fig. 3. The parameters of the model are set as follows: m1 = 26 mm, n1 = 10.8 mm, w = 2.8 mm, d1 = 6.8 mm, d2 = 6 mm, and g1 = 0.2 mm. When d1 and d2 have different values, the simulated transmission coefficient S21 appears as in Fig. 4. When d1 = d2 = 6 mm, the simulated S21 response has a -10-dB stopband bandwidth from 4.80 to 5.43 GHz. When d1 = d2 = 6.8 mm, the simulated S21 response has a -10-dB stopband bandwidth from 4.26 to 4.81 GHz. When d1 = 6.8 mm and d2 = 6 mm, the -10-dB stopband bandwidth of the transmission coefficient S21 is from 4.41 to 5.22 GHz. According to the simulated data, the widest stopband bandwidth occurs for the case when d1 = 6.8 mm and d2 = 6 mm. The obtained stopband also covers the harmonic passband frequency range.

To validate the software models, experimental prototype filters were fabricated and characterized. These prototype filters, with and without the DMS, are shown in Fig. 5. The measured parameters for the experimental bandpass filters are shown in Fig. 6. The -10-dB S11 bandwidth without the DMS is from 2.15 to 2.51 GHz; the passband insertion loss is less than 0.9 dB is from 2.16 to 2.53 GHz. There is a harmonic passband at 4.93 GHz and the S21 forward transmission response at the harmonic passband frequency is -11.4 dB. After adding the DMS, the insertion loss was found to be less than 0.9 dB from 2.23 to 2.56 GHz. The harmonic bandpass response was suppressed and the S21 response within the harmonic bandpass range is reduced to -40.7 dB at 4.93 GHz. The -30-dB stopband bandwidth of the bandpass filter with the DMS is from 3.34 to 6.50 GHz.

In summary, a bandpass filter was designed with meandered parallel coupled-lines to achieve a small size, and two non-uniform DMS units were loaded to suppress the bandpass filter's harmonic passband. The added DMS units proved effective in eliminating the harmonic passband at 4.93 GHz and widening the stopband bandwidth, achieving a -30-dB stopband bandwidth from 3.34 to 6.50 GHz. The visual evidence of the positive effects of adding DMS units to an existing filter design can be seen in the plots of Fig. 6, which show the forward and reflected scattering parameters for filters fabricated with and without the DMS units.

References
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