Application design of new CMRC broadband low-pass filter

　　1 Introduction

　　With the development of electronic technology, multi-frequency operation is becoming more and more common in radar, microwave, communication and other application systems, and the requirements for separation frequency are also increasing accordingly, requiring the use of a large number of high-performance filters. Therefore, filters are widely used in microwave and millimeter wave circuits. In low-frequency applications, lumped parameter filters have good performance, but as the frequency increases above the microwave frequency band, the Q value of lumped parameter components (capacitors, inductors) drops sharply, causing the insertion loss of the filter to be too large. At this time, distributed parameter components must be used to replace lumped parameter components, but the size of distributed parameter component filters is generally large, so it is necessary to reduce the size of microwave millimeter wave circuit filters.

　　In 2000, Professor Xue Quan of City University of Hong Kong proposed a compact microstrip resonator (CMRC), and then spiral compact microstrip resonator (SCMRC) and linear compact microstrip resonator (BCMRC) were proposed. However, the stopband range of the SCMRC structure is small (5.2GHz-7.6GHz), and BCMRC usually requires several units to achieve good low-pass characteristics due to its unsatisfactory attenuation characteristics in the stopband range. In response to these problems, this paper proposes a new CMRC broadband low-pass filter, whose maximum insertion loss is 0.3dB in the low-pass frequency range of 0-7GHz, the stopband frequency range below -10dB is 8.5GHz-22.1GHz, and the stopband frequency range below -20dB is 9GHz-20.8GHz. It can be seen that the filter has very low insertion loss in the passband and good attenuation characteristics in the stopband.

　　2 Structure and equivalent circuit

　　The new CMRC planar structure proposed in this paper is shown in Figure 1, and its LC equivalent circuit model is shown in Figure 2. The dielectric substrate uses Taconic CER_10, whose dielectric constant er=9.5 and thickness is 0.64mm.

　　Figure 1 Planar structure of CMRC

　　Figure 2 LC equivalent circuit model

　　3 Filter Characteristics Simulation Analysis

　　3.1 Influence of main structural parameters on transmission characteristics

　　We used HFSS to model and simulate the CMRC structure shown in Figure 1, and analyzed the influence of the main structural parameters on the filter transmission characteristics. In the simulation, we found that x1, y1 and y2 have a greater influence on the filter transmission characteristics, and their influence characteristic curves are shown in Figures 3 to 5. It can be seen from Figures 3 and 4 that reducing x1 and y1 can reduce the resonant frequency, thereby correspondingly reducing the low-pass frequency range. This is because in the equivalent circuit model, reducing x1 or y1 can increase the distributed series inductance per unit length (L0 and L1). As for the resonant frequency, it can be seen that the increase of L0 and L1 will correspondingly reduce the resonant frequency.

　　Figure 5 shows the effect of y2 on the transmission coefficient. The closer the value of y2 is to y3, the lower the resonant frequency. This is because increasing y2 can increase the distributed capacitance per unit length of the microstrip part. In addition, the reduction of the gap also increases the edge coupling capacitance, which leads to an increase in its equivalent capacitance C1, thereby reducing its resonant frequency and the low-pass frequency range.

　　Fig.3 Transmission coefficient changes with structural parameter x1

　　Fig.4 Transmission coefficient changes with structural parameter y1

　　Fig.5 Transmission coefficient changes with structural parameter y2

　　3.2 Optimization results and their main parameters

　　After the above analysis, the CMRC broadband low-pass filter is optimized and a good output S parameter characteristic curve is obtained as shown in Figure 6. The maximum insertion loss in the low-pass frequency range of 0 to 7GHz is 0.3dB, the stopband frequency range below -10dB is 8.5GHz-22.1GHz, and the stopband frequency range below -20dB is 9GHz-20.8GHz. It can be seen that the broadband low-pass filter has good low-pass and stopband attenuation characteristics. At this time, the main microstrip distribution parameters are given in Table 1.

　　Table 1 Main microstrip structure parameters

　　4 Conclusion

　　The filter has low insertion loss in a wide passband range and good attenuation characteristics in a wide stopband range. It can be used as an intermediate frequency filter for high- and medium-frequency mixers and can effectively suppress the local oscillator and its harmonic components. In addition, the broadband low-pass filter is very small in size and has the advantage of easy integration.