To improve the stopband frequency response of the filter, the DGS resonators were modeled and simulated with Microwave Office. Variations in the length of the horizontal arm (l) will shift the cutoff frequency and the attenuation pole location in the frequency domain. To understand the effects of changing the length of the horizontal arm, other dimensions of the structure were kept constant while the length (l) of the arms was varied (Fig. 9); simulation results are shown in Fig. 10. Decreasing the arm length resulted in decreasing the total capacitance, with the attenuation pole location moving to a lower frequency, and clear improvements in the reject-band response and sharpness of the cutoff-frequency response. The filter’s dimensions (w0 = 1.92 mm; w1 = 8 mm; w3 = 3 mm; h = 10 mm; l = 2 mm; and t = 2 mm) were computed and optimized with the aid of Microwave Office and Tex-line.
A meandered feedline has been used to previously to meet requirements for a compact polynomial five-deg. LPF with a quasi-elliptic approach and to also improve its rejection-band response (see Fig. 11). Addition of the meander line increases the inductance of the structure, which shifts the pole frequency to a lower-frequency range. Figure 12 shows strong suppression of the S21 response at higher frequencies, indicating that this modified resonator/filter topology affects the rejection band without any modifications in the size of the structure.
Figure 13 shows simulated EM field distributions for the DGS LPF in the stopband and passband. In the stopband at the 2-GHz (transmission zero) pole frequency, maximal magnet current concentrates around the first resonator, indicating that energy is not transferred to the output. As a result, the filter provides a stopband response [see Fig. 13(a)]. In the passband at the reflection zero of 1.2 GHz, almost all RF power passes through the filter structure. Maximum current concentration can also be observed on the metal stubs as well as on the microstrip line section between the three compensating capacitances on the top layer, which are coupled with both DGS resonators in the ground plane [see Fig. 13(b)].
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