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Abstract
A conventional interdigital bandpass filter (BPF) is characterized by coupled and tapped lines and affords low insertion loss (IL) and easy fractional bandwidth (FBW) adjustment. However, the maximum FBW of the filter is limited to 30%, beyond that, its gap size increases, thereby rendering filter fabrication impractical on a standard printed circuit board. In addition, the filter size cannot be changed because it dictates the operational frequency of the filter. Hence, in this study, we propose a compact interdigital BPF based on a spiral and folded stepped impedance resonator (SIR), which affords low IL and excellent group delay. The spiral, folded structure facilitates drastic FBW adjustment: the center frequency and adjustable range of the FBW of the designed BPF are 800 MHz and 80 to 180%, respectively. Additionally, the proposed BPF can adjust the FBW by k-factor which can adjust from 80 to 180%. The insertion and return losses of the proposed filter are 0.043 dB and 17.1 dB, respectively, and the group delay is 0.098 ns. The total filter size is only 13.8 mm × 5.98 mm, which corresponds to a size reduction by factors of >2/8 relative to a conventional filter and 2.1 relative to the latest BPF design. The group delay difference between the BPF and other filters is 0.15 ns. In addition, the range of adjustable FBW for the filter is 1.36 times different than for other filters.
Highlights
The demand for portable wireless communication systems is continuously increasing due to the rapid development of fourth industrial revolution technology
Such an interdigital bandpass filter (BPF) is impractical because the minimum gap size needed for printed circuit board (PCB) fabrication is 0.12 mm, and this size is extremely large for practical use
We note that, while the fractional bandwidth (FBW) of the conventional interdigital BPF can be sufficiently increased by adjusting lt and sj [2], it is practically limited to a maximum of 30% [2] because the maximum FBW of the J−inverter in the interdigital BPF is 30%
Summary
The demand for portable wireless communication systems is continuously increasing due to the rapid development of fourth industrial revolution technology. The compact size of a bandpass filter has increased to 180% of bandwidth [3,4] In this regard, microstrip BPFs offer beneficial features such as frequency selection, low insertion loss (IL) and return loss (RL), and excellent coupled−line characteristics. Interdigital BPFs, based on coupled structures, can be used to enhance IL, RL, and frequency characteristic performances; many studies have focused on reducing the size of interdigital filters for practical applications. BPFs with adjustable bandwidth and high performance using the bridged T−coils have been studied; such filters use a metal–insulator–metal (MIM) capacitor structure, which requires additional process masking and fabrication, thereby increasing manufacturing costs [24,25]. The filter is applied to analyze the effects of the tapped line on the feeding line (k-factor), with the potential to adjust the FBW
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