# Switchable Filter with Four Operating Modes Implemented on a 50-Ohm Microstrip Line
## 4 Switchable Filter Design with Bais Circuit
(**c**) Photograph of the fabricated switchable filter. Figure 14 shows the electric field distribution when the inductor values used in the bias circuit are 29 nH and 90 nH/68 nH, respectively. At this time, the simulation is performed at \(f_{1}\). In the case of 29 nH, it can be confirmed from the simulation results that the electric field is also formed in the bias circuit. On the other hand, when a 68 nH inductor was used in the V2 section and 90 nH inductors were applied to the other two bias lines, the amount of RF signal leaking into the bias circuit was significantly reduced, as confirmed by the simulated electric field distribution. Based on these facts, it can be confirmed that the small inductor value is not sufficient to operate as an RF chock to prevent the electric field from passing through to the bias circuit. Figure 14: E-field distribution at \(f_{1}\) when the inductor is (a) 29 nH, (b) 90 nH and 68 nH. Figure 13: Simulation results of switchable filter according to the values of resistors and inductors used in the bias circuit. (a) \(S_{11}\). (b) \(S_{21}\). Figure 15 shows the simulation results of the effect of the bias circuit applied position \(L_{i1}\) on the filter performance. As the bias applied position changes, there is little effect in other modes, but the effect is very strong in the BSF mode where the \(D_{2}\) diode is turned on. From the simulation results, as \(L_{i1}\) increases, the insertion loss of \(S_{21}\) tends to decrease in both resonant frequency bands. However, when \(L_{i1}\) is 5 mm, there is a problem of \(S_{21}\) ripple occurring in the 5.5 GHz band. Considering these facts, \(L_{i1}\) is selected as 3 mm. The switchable filter operates as two types of dual-band BPF, wideband BPF, and all-pass filter depending on various bias combinations. The simulation and measurement results for these four cases are presented in Figure 16. It is confirmed that some measured TZ occurred at a slightly lower frequency than the simulation. These inconsistencies may be attributed to slight inaccuracies in the lengths of the L- and C-shaped resonators caused by fabrication tolerances during the manufacturing process. In addition, the resonators are connected through PIN diodes, whose parasitic elements can alter the effective electrical lengths of the resonators. Considering these facts, the simulation and measurement results can be seen to be in good agreement overall. When \(D_{1}\) is off, \(D_{2}\) is on, and \(D_{3}\) is off, the switchable filter operates as a DB-BPF, and the results are presented in Figure 16a. The measured 3 dB fractional bandwidths centered at \(f_{1}\) = 1.4 and \(f_{2}\) = 5.1 GHz are 23.49 and 14.42%, respectively. The measured insertion losses at this time are \(-\)1.33 and \(-\)1.3 dB, respectively. On the other hand, when \(D_{1}\) is off, \(D_{2}\) is off, and \(D_{3}\) is off, the switchable filter operates as a DB-BPF, and the operating bandwidths are \(f_{1}\) = 2.4 and \(f_{2}\) = 4.2 GHz, respectively, and the measured 3 dB fractional operating bandwidths are 23.33 and 13.27%. The insertion losses are \(-\)0.88 and \(-\)2.04 dB, respectively. Figure 16c shows the result when the switchable filter operates as a wideband BPF. At this time, the bias conditions are \(D_{1}\) on, \(D_{2}\) off, and \(D_{3}\) off. The measured 3 dB fractional bandwidth is 73.36% (2.05-4.41 GHz) and the insertion loss is \(-\)0.69 dB. Figure 16d shows the result when operating as an APF. At this time, the bias conditions are \(D_{1}\), \(D_{2}\) don't care, and \(D_{3}\) on. The measurement results show that the APF exhibits an insertion loss of \(-\)2.34 dB in the entire operating frequency range of 0-7 GHz, confirming that signal transmission is efficient. Table 2 presents the performance comparison between previous switchable filters and the filter proposed in this work. Figure 15: Simulated filter performance with different bias positions \(L_{i1}\). Figure 16: Simulated and measured results.