# Switchable Broadband-to-Tunable Narrowband Magnetic Probe for Near-Field Measurements
## 3 Biasing Circuit  
This work has two active elements that need biasing; the PIN diode and the varactor diode. The selected PIN diode SMP1320-079 LF has a slight forward resistance \(Rf\)= 0.9 \(\Omega\) at 10 mA and a reverse biasing capacitance \(Cr\) = 0.3 pF. On the other hand, the varactor diode type SMV2201-040 LF we utilized has a capacitance range \(C\) = (0.23-2.22 pF). Figure 19 presents the biasing circuit for both the PIN and varactor diodes. Both DC current and RF currents will share the same feeding line. Therefore, three essential passive elements are involved to ensure the compatibility of these two sources. The \(Lc=10\) nH chocking inductor of prevents radiation leakage from passing through the biasing circuit since it works as a low-pass filter. A 100 nF blocking capacitor C1 is necessary to protect the network analyzer from the biasing current. Finally, other 100 nF blocking capacitors C2 and C3 are mandatory to prevent the diodes' reverse biasing and protect them from the short circuit since they work as a high pass filter.  
Figure 19: The biasing circuit for PIN and varactor diodes.  
Figure 18: The fabricated shape of the magnetic probe from the Pragoboard company.  
The path connecting the via-1 and the via-2 is part of the ground plane layers and carries a DC current. However, this part is involved in the magnetic resonance process (part of patch-slotted gap resonators), which iswhy it is connected to the vias. Therefore, two 100 pF capacitors are employed to prevent the back DC current for each diode from passing through this radiated path and to protect the diode from the short circuit by the DC current.  
The PIN diode has two states, ON and OFF, to switch between the narrowband and broadband states. At the same time, the varactor diode has a tunable mode to shift the resonance frequency when the PIN diode is in the ON status. Table 2 lists the diode modes with a variable DC voltage supply.  
One of the main advantages of near-field measurements is that they can perform in an ordinary laboratory with the necessary tools, unlike the far-field tests that affect the environment, which is why the latter needs chambers. The RIGOL DSA875 Spectrum Analyzer with the RIGOL VB 1032 directional coupler is utilized to measure the S11 parameter with the help of a DC voltage supply for biasing, as shown in Figure 20. Moreover, the sensitivity is achieved using the Spectrum Analyzer with the help of the printed reference board presented in [31].  
Figure 21 presents the measured S-parameters for different switching modes. The resonance frequency bands achieved are three bands instead of four (missing a band at 3.75 GHz), which is due to the biasing voltage value (5 V) of band 3.75 GHz being close to that of band 3.5 GHz (4 V). However, the achieved measured band of 3.5 GHz has wider bandwidth than the simulation one due to the merging of these two adjacent bands, as demonstrated in Figure 21a.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**NO.** & **1** & **2** & **3** & **4** & **5** \\ \hline VR/V & 0 & 4 & 5 & 12 & 20 \\ CAPACITOR/pF & – & 0.8 & 0.7 & 0.3 & 0.23 \\ PIN MODE & OFF & ON & ON & ON & ON \\ VARACTOR & OFF & M1 & M2 & M3 & M4 \\ MODE & & & & & \\ STATE/GHZ & 1–6 & 3.5 & 3.75 & 4.8 & 5.2 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Lists the diode modes with a variable DC supply.  
Figure 20: S11 parameter measurement with different biasing voltages.  
Moreover, the sensitivity of the broadband state at VR = 0 V is lower than the sensitivity of the broadband probe in Figure 9b. The reason is that adding the PIN and varactor diodes will add some losses.