Band-pass limiting filter based on microstrip comb line structure
Yuan.Xin.Li1, Ming.Tuan.Lin1, Ji.Bin.Liu1, and Shi.Xiong.Deng 1,2
1 College of Electronic Science, National University of Defense Technology, Changsha, China
2 Department of Microwave Integrated Circuits, Hebei Semiconductor Research Institute, Shijiazhuang, China
Email: leeyx2020@qq.com.
A band-pass limiting filter (BPLF) based on a microstrip comb line loaded with PIN diodes is proposed in this letter. In contrast to conventional comb line filter designs, the BPLF utilizes PIN diodes to replace capacitor components. When the power of the input signal is below a certain level, the proposed BPLF acts as a filter. While for an input signal whose power is above a certain level, it behaves like a limiter. According to measurement data, the proposed BPLF has a broad passband with an insertion loss of about 3.5 dB from 8.2GHz to 9.0GHz, and a low limiting level of less than 17dBm with a high power input of 50W (47dBm). In general, the proposed BPLF enables dual selectivity of frequency and power, and can be applied to the front end of RF channels.
Introduction: Filters and limiters are both important components in radio frequency (RF) systems, to achieve frequency filtering [1] and protection performance [2], respectively.
In general, filters are designed to allow in-band high power signals through with low loss, so that sensitive devices in the back end may be damaged. On the other hand, limiters based on PIN diodes are designed to have ultra-wideband [3], low insertion loss or integrated with amplifier [4], rather than high selectivity. Therefore, in order to ensure frequency selection and prevent the damage, the filters and limiters are cascaded together. However, it is costly and not easy to integrate and reduce insertion loss if the cascade is adopted directly in front of the RF receiver as usual.
Here we proposed a basic concept of integrating the design of filters with limiters based on the basic comb line bandpass filters, which is abbreviated to BPLF. As a kind of classic band-pass filter structure, microstrip comb line [5], is easy to design and implement. It is comprised of an array of coupled resonators in parallel, which are short-circuited at one end, and with lumped capacitances loaded between the other end and ground. Due to the presence of lumped capacitances, the resonators can be shortened thereby reducing area of the structure. In addition, it is possible to design reconfigurable filters by controlling the value of the loaded capacitances or adjusting lumped elements, such as frequency-tunable [6-8] and switchable filters [9].By replacing lumped capacitors with PIN diodes, filtering performance under small signals and protection effect under large signals can be achieved together with a more compact size and a lower insertion loss compared to the cascade design. It can be used to deal with intentional or unintentional electromagnetic interference in complex electromagnetic environments.
Principle and design:Fig.1 shows the principle of the proposed BPLF. The topology in the middle part was modified from the equivalent circuit of a comb line filter [10]. The key point of the modification is to replace the shunt capacitor with a PIN diode (inside the blue box in Fig.1). Under the input with low power, the diodes are equivalent to capacitances in the turned-OFF state (inside the green box). Thus, the circuit shows the characteristics of a bandpass filter. On the contrary, if the power of input electromagnetic signals exceeds the threshold, diodes work in a turned-ON state and is equivalent to a resistance approximate to zero (inside the red box). Therefore, the switching process of PIN diodes can be used to realize limiting properties. The other parts of the topology include transformers at input and output, parallel shorted shunts, and J inverters. and in Fig.1 represent admittance of the terminating line and parallel stub, respectively, the same as the comb line filter equivalent circuit.