# Switchable Filter with Four Operating Modes Implemented on a 50-Ohm Microstrip Line
## 1 Introduction  
As wireless technology convergence accelerates, the demand for radio frequency (RF) front-end components that can support multiple communication standards within a single platform is increasing [1]. The 5G NR Sub-6 GHz band requires a wide bandwidth of up to 100 MHz for high-speed data transmission [2]. In contrast, IoT services such as NB-IoT and IEEE 802.11 operate in a narrow bandwidth of several MHz to tens of kHz to achieve very low energy consumption [3]. If these heterogeneous requirements are addressed with multiple circuits, the circuit area increases, which is disadvantageous for terminal designs where miniaturization is important. Accordingly, switchable filters that provide various frequency responses in a single circuit are attracting attention. Among various implementation methods, PIN diode-based reconfigurable filters are particularly suitable for multi-standard transceivers that must quickly switch between wideband 5G links and ultra-narrowband IoT channels due to their fast switching speed and simple circuit configuration [4, 5]. In [6], a switchable filter with four distinct modes was proposed by integrating a bandstop filter (BSF)/all-stop filter (ASF) structure with a dual-mode resonator (DMR) in a parallel configuration. In [7], three modes of bandpass filter (BPF), BSF, and DB-BPF were implemented based on coupled-lines. In [8], a switchable filter capable of implementing two BPFs and one BSF was proposed, while in [9], a switchable filter designed to implement three modes--BPF, DB-BPF, and BSF--was proposed by combining one main path and an additional path. In [10], a PIN diode was used to switch between BPF and BSF, and a varactor diode was used to control their resonant frequencies. Similarly, in [11], a structure enabling switching between two modes, BPF and BSF, was proposed, with an absorptive circuit added to enhance the matching characteristics of both modes. In [6, 10], two or morestructures with different filter functions are positioned in parallel, and they operate in different modes depending on which structure is activated. Since sufficient length must be guaranteed for coupling between each structure, it has the disadvantage of being large in size and complex in design parameters. In the case of [7; 8; 9; 11], a method is used to switch the filter mode by selecting one of several lines through a single pole multi-throw switch. However, this structure is larger than the method that utilizes coupling because it arranges multiple structures in parallel. In addition, since it is a method of controlling multiple PIN diodes, the complexity of the bias circuit increases and the insertion loss increases. To address these issues, research on implementing filter structures directly on 50 \(\Omega\) microstrip lines has been attracting attention [12; 13; 14; 15]. In [12], a sext-band BPF was proposed by adding a symmetrical short stub outside the 50 \(\Omega\) line. In [13], a filter capable of switching between three modes--a broadband filter, a dual-band filter, and a tri-band filter--was implemented by integrating a stub with a PIN diode into the 50 \(\Omega\) transmission line. However, both studies had the problem of increasing the overall filter size due to the long stub. In [14], a structure capable of switching between BPF and DBPF was proposed using a switchable J-inverter. However, it still had the problem of large size, with a large IL (Insertion Loss) of 2.9-4.9 dB using 13 diodes. In [15], a dual-band notch filter with tunable frequency was implemented by applying an embedded split ring resonator (ESRR) structure loaded with a varactor inside a 50 \(\Omega\) line. In [16], a simple and compact quadruple-band pass filter was implemented by inserting a C-shaped open-loop resonator inside a 50 \(\Omega\) line, but the spacing between resonant frequencies is fixed, so it has limitations in operating in various environments.