Wei Kang, Miao Chen, Yang Guo, and Wu Wen

High filter selectivity is desirable for rejecting unwanted spurious signals and for separating multiple channels in communications systems. Passband selectivity can be improved through the placement of transmission zeros (TZs) at appropriate frequencies, without sacrificing filter passband performance. To demonstrate this approach a novel microstrip dual-mode bandpass filter with multiple controllable TZs was designed, simulated, and fabricated. The location of the TZs can be controlled independently using dual-behavior resonators (DBRs) consisting of two pairs of open-circuit stubs. The filter stopband selectivity performance can be adjusted in this manner while achieving enhanced coupling strength between the feedline and the resonator. A prototype passband filter was designed and constructed with passband centered at 17.3 GHz and 2.8% fractional bandwidth, using TZs at 9.0, 16.3, and 18.3 GHz.

Dual-mode microstrip filters are widely used in microwave wireless communications systems due to their capabilities of providing low-loss, narrow passbands with small size and low mass.1,2 In a dual-mode bandpass configuration, dual-mode microstrip resonators are generally fed by a pair of orthogonal feed lines arranged at 90 deg. to produce the two degenerate modes and to couple to each other. However, orthogonal feed lines may not be physically suitable for many practical applications. Alternately, a dual-mode microstrip filter may be fed by a pair of feed lines arranged at 180 deg. geometrically along a straight line, as shown in Fig. 1. The nonorthogonal input/output (I/0) ports are arranged to enhance the flexibility of the circuit layout.

According to ref. 7, the positions of the TZs can be adjusted by perturbation size. To explore this, simulated responses for different values of perturbation size are plotted in Fig. 2. The TZs in the simulation are closely associated with the different perturbation sizes; however, the filter passband characteristics are significantly impacted, and the TZs cannot be adjusted independently.

As an alternative to this approach, a different method of realizing a microstrip dual-mode bandpass filter with multiple controllable transmission zeros is proposed. The geometry for this novel filter is shown in Fig. 3. The input/output feed lines are connected to two pairs of open-circuited stubs which exhibit both coupling and tuning effects. When the size of the patch perturbation is fixed, the filter's three TZs can be adjusted by modifying stub lengths L1, L2, and L3, respectively. This is demonstrated by means of computer simulations in Fig. 4 for different microstrip lengths L1, L2, and L3. As can be seen, the locations of the TZs can be shifted independently by tuning stub lengths L1, L2, and L3; meanwhile, the passband frequency response is not affected. Comparing this new filter design with conventional dual-mode filter methods,4-7 this new filter provides dual-mode characteristics with the benefits of enhanced coupling strength and multiple adjustable TZs at finite frequencies.

To demonstrate effectiveness of this design approach, a dual-mode bandpass filter was designed and fabricated on RT/duroid 5880 substrate from Rogers Corp. with relative dielectric constant of 2.2 and thickness of 0,254 mm. After performing simulations of the response of the microstrip filter using IE3D three-dimensional (3D) electromagnetic (EM) simulation software, available from Mentor Graphics, accompanies by elaborate tuning, the circuit dimensions shown in the table were determined for the prototype filter design. The prototype filter is shown in Fig. 5 to illustrate its miniature dimensions. Figure 6 shows simulated and measured frequency responses of the prototype dual-mode filter. Measurements on the prototype filter reveal insertion loss of 2.6 dB at a center frequency of 17.3 GHz, including the insertion loss of the SMA connectors. The fractional bandwidth is about 2.8%. The simulated responses for the filter are in good agreement with the measurements, using the three TZs at 9.0, 16.3, and 18.3 GHz.

In summary, it was possible to achieve excellent performance by designing a dual-mode microstrip filter with a BDR structure. By means of two pairs of open-circuit stubs, the dual-mode bandpass filter exhibits multiple TZs at its lower and upper stopbands. The values and locations of the TZs can be controlled, and it is possible to realize a range of selectivity values in the stopband.

Acknowledgment

This work was supported by the State Key Laboratory of Millimeter Waves, Southeast University, Open Research Program, under contract No. K200923.

References

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