A Filter Synthesis Method for Multi-Band Filter Design

Abstract

In this paper, we presented a new LC prototype synthesis method for the multi-band filter. For synthesis a multi-band filter with the required frequency response, we proposed the diagram of poles and zeros, also, we proposed the optimization process for finding the combination of optimized poles and zeros. From the transfer and reflection functions calculated from poles and zeros, we performed the quasi-elliptic LC prototype synthesis of multi-band filter. Using the proposed LC prototype synthesis method of multi-band filter, dual-band filter operating at GSM(880～960 MHz) and ISM(2,400～2,500 MHz) and triple-band filter operating at GSM(880～960 MHz) and ISM(2,400～2,500, 5,725～5,850 MHz) were designed and fabricated.

A Method for Improving Stopband Characteristics of a Dual-Band Filter

“…The filter was fabricated from a 0.76 mm thick substrate with a relative di- electric constant of 3.5. The total filter size, excluding feed lines, is 14.6×13.2×0.76 mm, which is a very compact size compared with the conventional filter described in [7], [8]. Table 1 shows a comparison of previously reported dual-band filters.…”

“…However, the passband characteristics are not significantly affected by the electric couplings because Cmo and Cmi are very small. The effects on Y odd and Y even , which were studied in [8], [9], are also small. Therefore, we can achieve improved stopband rejection characteristics with negligible effects on the passband performance.…”

“…7 shows the proposed dual-band filter with improved stopband characteristics. The filter has a very simple structure, which is designed using microstrip quasilumped elements [8], [9]. The electric couplings are realized by the distance control between the symmetric open stubs of the filter structure.…”

“…Fig. 1(a) shows the lumped-element circuit model of the dual-band filter that was previously studied in [8], [9]. This dual-band filter has two transmission zeros at the resonance frequencies of Lr1, Cr1 and Lr2, Cr2.…”

“…Microstrip cross-coupled structures were also proposed to realize the coupling matrix [7], but these filters showed high insertion losses, which resulted from the finite Q factor of resonators, and have a large size. For these reasons, we proposed an LC prototype for multi-band filters [8], [9]. This method has a simpler design process because it can be directly synthesized from the poles and zeros.…”