Improving the Coupling Characteristics of Bandpass Filters by Using Multilayer Structure and Defect Ground Units

Abstract

This paper proposes a novel multi-layer microstrip 2.4 GHz bandpass filter fabricated on two bonded substrates. The multi-layer filter has a symmetrically-bent, open-loop, squarering stepped impedance resonator (SIR) structure on the top surface of the upper substrate, a similar SIR pattern rotated through 900 on the lower surface of the upper substrate, and a pair of L-shaped symmetrical defected ground units on the bottom surface of the lower substrate. The simulation results demonstrate that the multi-layer structure improves the electromagnetic coupling characteristics of the bandpass filter and increases both the return loss by more than 30 dB at the center frequency compared to that achieved using a conventional two-layer (SIR-ground) structure. In addition, the defected ground units etched onto the metallic ground plane beneath the feed lines yield a significant reduction in the spurious harmonics at a frequency of 5.2 GHz. Experimental results are presented to show the performance of the proposed filters.

Theory and Realization of Simple Bandpass Filters With Antiparallel Configuration

“…, while ω Z1 is given by (11). Expressions (26) and (27) give m and n values that produce transmission zeros ω EZ 1 and ω EZ 2 on arbitrary angular frequencies within the ranges: ω Z1 ≤ ω EZ1 ≤ 1 and 1 ≤ ω EZ2 ≤ ∞.…”

“…The isolation in the upper stopband increases monotonically with the frequency increase as well as with the capacitor values increase. The lower stopband contains two S 21 zeros: ω Z0 = 0 and ω Z1 given by (11), as well as a local minimum of the isolation between them (marked with M (i) -M (iv) ). Figure 3.…”

“…Some papers show that the filter's characteristics are a consequence of general network configurations, like [3] that demonstrated that the parallel connection of the two port networks has features that can be used for designing microwave BPFs, or [4] that shows that the using of a 0 • feed structure creates two extra transmission zeros at the frequencies close to and on the opposites of the passband. A conclusion that some distinctive characteristics of the filters, such as the number and positions of the transmission and reflection zeroes, are a consequence of their topology, can be reached based on the similar characteristics of BPFs described in [5][6][7][8][9][10][11][12][13][14], distinguished by the authors into different kinds and analyzed and designed by different methods. The correlation between the topology and the electrical characteristics is especially noticeable for BPFs with capacitive coupled resonators [15][16][17][18][19][20][21][22].…”