# Switchable Filter with Four Operating Modes Implemented on a 50-Ohm Microstrip Line
## 4 Switchable Filter Design with Bais Circuit  
Figure 11(a) shows a switchable filter with five diodes and a bias circuit to operate them. The configuration of the switchable filter is the same as Figure 1 except for the bias circuit and diodes. The two \(D_{2}\)s located on the left and right gaps and the two \(D_{3}\)s located on the upper and lower outer gaps are each configured as a pair. The bias circuit consists of an inductor (green), a capacitor (blue), and a resistor (black). As is well known, the thinner and shorter the bias line, the better. Accordingly, the line width was set to 0.1 mm and the length to 5.6 mm. To implement the bias circuit in a limited space, the capacitor is located on the back side where the GND is located. \(L_{i1}=3\), \(L_{i2}=1\) mm are set, and other parameters are maintained as optimized values obtained based on the parameter study performed in Section III. Figure 11(b) shows the equivalent circuit of the diode used. It acts as a resistor in the On state and as a capacitor in the Off state. \(R_{on}\) is 5.2 ohm, \(C_{off}\) is 0.025 pF, and \(L_{5}\) is about 0.5 nH. When simulating the switchable filter including the bias circuit, the equivalent values of the diode are all considered. The bias voltages used to control the On/Off state of each diode are shown in Table 1. In the case of \(D_{3}\), the bias voltage is applied by connecting a bias tee to the ports at both ends of the line. The voltages applied to the bias tees are indicated as \(V_{4}\) and \(V_{5}\) in Figure 11(a), respectively. Figure 11(c) shows the fabricated switchable filter. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline  & \multicolumn{3}{c}{**Condition [V]**} & \multicolumn{5}{c}{**Diode State**} & \multirow{2}{*}{**Operating Mode**} \\ \hline \(V_{1}\) & \(V_{2}\) & \(V_{3}\) & \(V_{4}\) & \(V_{5}\) & \(D_{1}\) & \(D_{2}\) & \(D_{3}\) & \\ \hline
4 & 1 & 0 & 0 & 1 & On & Off & Off & Wideband \\
1 & 3 & 6 & 0 & 1 & Off & On & Off & Dual-band \\
2 & 4 & 0 & 0 & 1 & Off & Off & Off & Dual-band \\ - & - & - & 1 & 0 & - & - & On & All pass \\ \hline \end{tabular}
\end{table} 
Table 1: Applied bias voltage conditions and operating modes. Figure 11: Simulation results of Type 3 according to the change in length \(L\). Figure 13 shows the simulated filter characteristics when the values of \(R\) and \(L\) used in the bias circuit are changed. The values of \(R\) and \(L\) used in the bias circuit have a large effect when the proposed structure operates as a dual-band filter but have a relatively small effect when it operates as a wideband filter. For this reason, only simulation results applying the bias circuit to the dual-band filter are presented in this paper. As the inductor value increases, the insertion loss in the 1.5 GHz band, which is \(f_{1}\), tends to decrease. At this time, the insertion loss in the 5.5 GHz band, which is \(f_{2}\), showed almost no change. Since the inductor has a large impedance value as the frequency increases, the impedance is relatively large in the \(f_{2}\) band, which is a relatively high frequency. Accordingly, the RF signal flowing through the bias circuit can be appropriately blocked. On the other hand, the impedance is relatively small in the \(f_{1}\) band, so some RF signals flow into the bias circuit. This is because the insertion loss in \(f_{1}\) improves as the inductor value increases. However, when the inductor value is 90 nH, \(S_{21}\) ripple is formed in the \(f_{2}\) band. Among the three inductors, the insertion loss could be reduced by lowering the inductor value of the \(V_{2}\) section to 68 nH. When no resistor is used in the bias circuit, the insertion loss is the smallest in the 1.5 GHz band, but resonance occurred at 0.6 GHz and strong ripple occurred in the 5.5 GHz band. When a 0.3 Kohm resistor is used, the insertion loss in the \(f_{1}\) band decreases, but the insertion loss in the \(f_{2}\) band increases. Figure 12: (**a**) Proposed filter with bias circuit included. (**b**) Equivalent circuit of the MADP-000907-14020 PIN diode used.