# A Filtering Switch Made by an Improved Coupled Microstrip Line
###### Abstract  
In this paper, we propose a new filtering switch with excellent working performance made using an optimized coupled microstrip line. Upon analyzing the RF (radio frequency) front-end's system structure, the switching device was simplified to a diode, which was connected to the microstrip circuit we designed to become a filter switch with both filtering and shutdown functions. First, we obtained an equivalent schematic of this filtering switch based on the relevant microstrip line theory. This switch consists of two coupled microstrip circuits, parallel-coupled feed lines and coupled-line stub-load resonators (CLSs), and a PIN diode. Second, the operating principle is described by the switching of the operating states, with ideal shutdown performance in the off state and considerable selectivity and excellent out-of-band rejection performance in the filtered state. Finally, a prototype filtering switch with a center frequency of 0.8 GHz was designed and tested. After subsequent optimization and improvement, the simulation and test performance results were noticeably consistent, consequently verifying the performance requirements of this filtering switch in two operating states in the center frequency band. filtering switch; coupled microstrip line; working performance +
Footnote †: journal: Applied  
0000-0001-9993-3300/0000-00001-993-3300/00001-993-3300/00001-993-3300/00001-993-3300/0001-993-33  
after designing a triplexer using the aggregate component theory [1] and integrated filter switches based on the fractal stub-loaded resonator (F-SLR) design [9]. In our paper, we combine a theoretical analysis of a coupled microstrip line and PCB fabrication process to design and produce a filtering switch with two functions of being switched between filter and off. Previously, many researchers have labored on the design and commissioning of filtering switches. The filtering switch proposed in [10] has a compact size and clear passband selectivity; however, it has a relatively high insertion loss (IL) in pass-state operation and less than optimal filtering operating performance. Although the view that the transmission zeros (TZs) generated by the filtering switch enhance the filtering switch's through-state selectivity to some extent was introduced in [11], the selectivity impact exhibited by using the filter switch in the filtered state is still relatively unsatisfactory. Although the filtering switch mentioned in [2] exhibits excellent shutdown rejection, it is not the best for use in the filter function, and while the filtering switch designed in [12] has excellent turn-off rejection performance and excessive integration, its relative bandwidth (FBW) is only 8.96%, with a connection insertion loss of 1.72 dB, which indicates an unsatisfactory band pass filtering effect. To avoid the above filtering switch problems and to fill knowledge gaps in the related fields, in this paper, we combine an improved filter response theory of coupled microstrip lines [13] with the basic design techniques of filtering switches [14; 15; 16]. We also studied related research in similar fields, such as MEMS switch-based filters [17], ferrite resonant filters [18], and enhanced gain in waveguide antennas [19]. Based on these theoretical approaches, we propose a filtering switch designed according to the filtering effect of coupled microstrip lines. Our design idea is to choose parallel-coupled feeder lines and coupled-line stub-load resonators (CLSs). At a wider operating bandwidth, the parallel-coupled feeder lines can reduce the insertion loss (IL) and return loss (RL), and the coupled-line stub-load resonators (CLSs) can produce symmetrically controllable transmission zeros (TZs). Compared to the design idea of using aggregate elements in [1], the design using these two coupled microstrip lines is easier to implement. There is no need to consider the specific length setting of the short route, and the design is less difficult.