## 3 Simulated and Measured Results
The over-voltage protection circuit, low-voltage LDO regulator, and charge control circuit of the RF-DC rectifier required 33 \(\upmu\)W, 5 \(\upmu\)W, and 4 \(\upmu\)W, respectively, to charge the battery to 24 \(\upmu\)W. The simulation results revealed that charging current was 16 \(\upmu\)A at N = 55, and the over-voltage protection mechanism was started at 2.002 by setting the output voltage of the over-voltage protection circuit, output voltage of the charger control circuit, and reference voltage at 1.7 V, 1.5 V, and 1.4 V, respectively. Figure 16 shows the simulated total power conversion efficiency of the proposed RF-energy-harvesting IC, which was termed the system _PCE_. The maximum system _PCE_ was 29.873% at an input power \(P_{in}\) of \(-\)12 dBm. The maximum rectifier _PCE_ was reduced to the same value at \(P_{in}=-\)12 dBm by integrating all designed circuits into a single chip (Figure 11). Table 1 summarizes the performance and compares it with that of other RF-DC rectifiers. The simulated maximum rectifier _PCE_ in this study was superior to the _PCEs_ of previously published RF-DC rectifiers. Table 2 presents the simulated specifications of the proposed RF-energy-harvesting IC. Figure 14: Simulated battery voltage and charging current of the charger control circuit. Figure 15: Layout of the proposed RF-energy-harvesting IC. The matching network was completed with off-chip components to have an input impedance of approximately 50 \(\Omega\). Figure 17 shows the matching network, which was designed to obtain not only maximum power transfer but also maximum conversion efficiency. The E5071C network analyzer was used to measure the input impedance against various input powers. Moreover, multiple sets of matching circuits were designed to ensure that each set of the matching circuit exhibited an input return loss (S\({}_{11}\)) of less than \(-25\) dB. For example, an input impedance of \(51.464-j\times 1.3071\) could be achieved with an input return loss (S\({}_{11}\)) of \(-34.271\) dB by putting a parallel inductance (L) of \(7.5\) nH and a series capacitance (c) of 22 pF on a printed circuit board (PCB). Figure 18 shows the measured input return loss in the Z-smith chart at an input power and RF frequency of 0 dBm and 915.0000 MHz, respectively. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Reference** & [19] & [28] & [29] & **This** \\
**(year)** & **2013** & **(2014)** & **(2017)** & **Work** \\ \hline Process & \(0.18\) μm & \(0.18\) μm & \(65\) nm & \(0.18\) μm \\ Input power & \(-7\) dBm & \(-21.2\) dBm & \(-17.7\) dBm & \(-7\) dBm \\ RF frequency & \(5.2\) GHz & \(925\) MHz & \(900\) MHz & \(915\) MHz \\ Rectifier stages & \(5\) & \(7\) & \(5\) & \(6\) \\ Matching circuit & On-chip & Off-chip & Off-chip & Off-chip \\ Maximum _PCE_ & \(42\%\) & \(43\%\) & \(36.5\%\) & \(43.6\%\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of performance and its comparison with those of previous RF-DC rectifiers. Figure 16: Simulated total _PCE_ of the proposed RF-energy-harvesting IC. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Designed Specifications** \\ \hline Technology process & TSMC 0.18 μm 1P6M CMOS process \\ RF frequency (\(f_{\text{in}}\)) & \(915\) MHz \\ Input power range (\(P_{\text{in}}\)) & \(-13\sim+20\) dBm \\ Output dc voltage (V\({}_{\text{dc}}\)) & \(1.773\sim 1.809\) V \\ Maximum _PCE_ of rectifier & \(43.603\%\) \\ LDO output voltage (V\({}_{\text{dd}}\)) & \(1.50\) V \\ Charging current (I\({}_{\text{bat}}\)) & \(16\) μm \\ Maximum _PCE_ of RF IC & \(29.873\%\) \\ Chip area (include pads) & \(0.58\times 0.49\) mm\({}^{2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Simulated specifications of the proposed RF-energy-harvesting IC. Figure 19 shows the measurement platform of the designed RF-energy-harvesting IC with capacitor, which includes the PCB (\(15.576\times 7.906\) cm\({}^{2}\)), network analyzer (Agilent E5071C), RF-IC (\(0.58\times 0.49\) mm\({}^{2}\)), Powercast transmitter (TX91501), and multimeter (CNN 38).