# A Survey of NFC Sensors Based on Energy Harvesting for IoT Applications
## 2 NFC Energy Harvesting
### NFC Reader Design Considerations  
Current mobiles recently incorporated NFC readers. NFC-based sensors are, therefore, normally read with these devices. However, certain applications (e.g., industrial ones) require a specific reader. Manufacturers of integrated circuits have solutions based on a low-cost single-chip reader. Such cases require the antenna for the reader and the control software of the microcontroller connected to the reader to be designed. To achieve maximum read distance, maximum power must be transmitted to the antenna; thus, the impedance of the transmitter must be the conjugate of the antenna. For this, \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Case** & **Inductance** & **Capacitance** & **Resonance** & **Antenna** & **Tuning Capacitance** \\  & **(\(\mu\)H)** & **\(C_{p}\) (pF)** & **Frequency (MHz)** & **Quality Factor** & \(C_{\text{tuning (pF)}}\)1 \\ \hline On air & 2.62 & 1.64 & 76.3 & 153 & 25.9 \\ On body & 2.62 & 9.02 & 32.7 & 6 & 18.5 \\ On body with ferrite foil & 3.34 & 6.70 & 37.5 & 131 & 9.5 \\ On body with a foam & 2.62 & 2.78 & 58.9 & 91 & 24.8 \\ space of 1 mm & 2.62 & 3.64 & 51.5 & 38 & 23.9 \\ On body with a plastic & 2.62 & 2.88 & 57.9 & 92 & 24.7 \\ \hline \hline \end{tabular} \({}^{1}\) Assuming \(C_{\text{IC}}\) = 25 pF.  
\end{table} 
Table 4: The equivalent circuit of the antenna and tuning capacitance as a function of the location. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Layer** & **Relative Permititivity** & **Conductivity (S/m)** & **Thickness (mm)** \\ \hline Skin & 120 & 0.25 & 1.5 \\ Fat & 38 & 0.21 & 4 \\ Muscle & 152 & 0.74 & 25 \\ Bone & 11 & 0.03 & 25 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Dielectric layers used in the simulation of the arm at 37 \({}^{\circ}\)C.  
a matching network must be designed. In addition to reducing electromagnetic (EM) interference with other systems, a low-pass band filter is inserted after the transmitter to attenuate the harmonics of the transmitted signal. This filter introduces some extra distortion into the signal and increases the bandwidth (or reduces the quality factor). A simple L-matching network is used as the matching network. In practice, the Tx output is usually differential (to enable double output voltage swing from a single supply voltage). Figure 11a shows a model of a differential reader with an EM interference (EMI) filter (capacitance \(C_{0}\), inductor \(L_{0}\), and inductor losses \(R_{0}\)), the matching network, which consists only of capacitances (\(C_{s}\) and \(C_{p}\)), and the antenna model. The antenna is assumed to be an inductive load (the antenna resonance frequency is higher than the operation band). This load impedance often falls within the allowed region on the Smith chart that can be matched with an L-matching network with two capacitors. The output transmitter resistance \(R_{out}\) depends on the transmitter's current consumption and, therefore, on the transmitted power, and is generally given by the IC manufacturer. HF capacitors (with C0G or NP0 dielectric) have negligible losses and less tolerance. The input impedance from the receiver is typically capacitive and is modeled as capacitance \(C_{in}\). The resistance \(R_{x}\) is inserted in order to attenuate the transmitter signal and to avoid receiver saturation. The antenna model can be obtained from electromagnetic simulations or from S\({}_{11}\) measurements with a VNA [27]. The \(C_{a}\) capacitance is derived from the antenna's unload resonance frequency. The design procedure is described below. 1. Design of EMI filter: a filter cut-off is chosen between 15-20 MHz and is given by  
\[f_{cut}=\frac{1}{2\pi\sqrt{L_{0}C_{0}}}. \tag{12}\]  
2. Adjustment of the maximum quality factor: the second step is to adjust the quality factor to make it equal to \(Q_{1 max}\). In both this step and the design of the matching network, it is assumed for the design criteria that the tag is far enough away to make the coupling very weak.