Design of ultra-wideband bandpass filter based on three-line coupling structure

　　1 Introduction

　　With the continuous development of communication technology, people have higher and higher requirements for the communication rate and communication quality of information systems. In this context, ultra-wideband technology (UWB) has become a research hotspot in the current communication field. In February 2002, the US Federal Commission authorized the use of the frequency band between 3.1GHz and 10.6GHz for UWB communication. As a result, the research on UWB bandpass filters, which are an important part of the communication system, has also made great progress. The literature proposes a type of broadband filter based on high-loss materials, which has a flat broadband characteristic, but the insertion loss is too large. The structure of resonant rings and open-circuit branches is used to realize ultra-wideband filters, but the return loss is only 10dB. The bandpass characteristics are realized by the combination structure of high-pass and low-pass filters. Parallel short-circuit branches are used to control out-of-band characteristics. In order to obtain low-loss and easy-to-process structures, multi-mode bandpass filters have been widely studied. In recent years, with the development of new materials and new structures, EBG structures have been introduced into the research of ultra-wideband bandpass filters.

　　This paper proposes a type of planar ultra-wideband filter using a microstrip three-line coupling structure and parallel short-circuited branches. The coupled transmission line is used to achieve broadband characteristics, and the short-circuited branches obtain good out-of-band suppression. The filter is simple in design, compact in structure, and has good in-band and out-of-band characteristics. The insertion loss is less than 1.5dB, the return loss is higher than 15dB, and the in-band delay is less than 0.3ns, which meets the communication requirements and can be widely used in microwave ultra-wideband communications.

　　2 Theoretical Analysis

　　This type of ultra-wideband bandpass filter consists of two parts. Parallel coupled lines and parallel short-circuited branches. We know that the λg/4 coupled transmission line can achieve bandpass characteristics. The two transmission zeros are located at f0 and 2f0 respectively. In order to obtain a larger bandwidth, we use three-line coupling instead of two-line coupling to increase the coupling strength. As shown in Figure 1, the 3-dB bandwidth is increased from 95% to 105%. Figure 2 shows the 3-dB bandwidth as the coupling line width (W1, W2) and the bandgap width (S) change.

　　(a) Schematic diagram

　　(b) Transmission characteristic S21 variation diagram

　　Figure 1 Two-wire and three-wire coupling structures

　　(a)

　　(b) (c)

　　Figure 2. 3-dB bandwidth as a function of (a) bandgap width, (b) coupling line width, and (c) coupling line width.

　　In order to obtain good out-of-band characteristics, we introduce two parallel short-circuit branches, one on each side of the coupled line. Figure 3 shows the transmission characteristics of the parallel short-circuit branches. We found that as the length increases, the band gap moves toward low frequencies; as the width increases, the band gap moves slightly toward high frequencies. Therefore, we can affect the bandwidth of the filter by changing the length and width of the short-circuit branches.

　　Figure 3. (a) Schematic diagram of a parallel short-circuit branch. Transmission characteristics as a function of (b) line width and (c) length.

　　3 Filter Design and Implementation

　　When designing the filter, we first select the appropriate odd and even mode impedance by changing the width of the coupled transmission line and the gap width; then select the appropriate length and width of the short-circuit branch to ensure that the band gap is around 12GHz. We use the electromagnetic simulation software Microwave office for principle circuit analysis and the electromagnetic simulation software CST for three-dimensional simulation. A polytetrafluoroethylene circuit board with a dielectric constant of 2.65 and a height of 1.0 mm is used for processing.

　　Figure 4: (a) Schematic diagram and (b) equivalent circuit of the designed ultra-wideband filter

　　Figure 4(a) shows the structural dimensions of the designed ultra-wideband bandpass filter, and the corresponding circuit equivalent diagram is given in Figure 4(b). The coupled transmission line acts like an admittance converter.

　　Figure 5: Comparison of the designed ultra-wideband filter (a) photo and (b) results

　　Figure 5(a) is a real photo of the designed ultra-wideband bandpass filter, and the corresponding measurement results are given in Figure 5(b). It can be seen that the test results are in good agreement with the simulation results, covering the entire communication range of 3.1~10.6GHz. The in-band insertion loss is less than 1.5dB, the return loss is higher than 15dB, the in-band group delay is less than 0.3ns, and the out-of-band characteristics are better than 15 dB.

　　4 Conclusion

　　This paper proposes a type of ultra-wideband bandpass filter using a three-line coupled structure and parallel short-circuited branches. This type of filter uses coupled transmission lines to achieve broadband characteristics and uses short-circuited branches to obtain good out-of-band suppression. Because of its simple design, compact structure, small size, easy processing, and good in-band and out-of-band characteristics, this type of filter is suitable for wide application in ultra-wideband communications.