# High-Efficiency Triple-Band RF-to-DC Rectifier Primary Design for RF Energy-Harvesting Systems
## 2 Radio Frequency Energy-Harvesting Design
### Dual-Band Rectifier Design  
The rectifier circuit is at the heart of an energy-harvesting module. It is responsible for converting the received RF power into DC voltage, sufficiently enough to charge a battery or feed battery-free low-power electronics. It is decisive in providing high values of RF-to-DC power conversion efficiency and operating in multifrequency bands to convert as much ambient RF energy as possible. One of the most established approaches to design an RF-to-DC rectifier is the Schottky diode-based rectifying circuit [23]. Figure 2 illustrates the design process of the proposed Schottky diode-based rectifier, as well as the main considerations that are taken into account. Based on Figure 2, the first steps of a rectifier's design were to select the appropriate circuit type and elements (Schottky diodes, resistors, capacitors). The Schottky diode, to which is attributed a low forward voltage drop and low substrate leakage, can improve the RF-to-DC conversion efficiency, as well as the DC  
Figure 1: Basic block diagram of a rectenna system. output voltage [24, 25]. Thereinafter, we calculated the S-parameters, the power conversion efficiency (PCE), and the DC output voltage of the system using the Advanced Design System (ADS-\(\copy\) Keysight Technologies 2000-2021). The variations of the rectifier's input impedance with the RF input power resulted in the use of the Large-Signal S-Parameter simulation tool of ADS. Depending on the outcomes, the next steps involved the tuning of the physical parameters of the impedance-matching network to adjust to a standard antenna port of 50 Ohm and the recalculation of the previously mentioned parameters. We repeated the same procedure for every frequency of interest until we met the desired criteria. Finally, we computed the efficiency and DC output voltage of the overall circuit as a function of the incident RF input signal. In this work, the HSMS285C (SOT-323) Schottky barrier diode, which has a low forward voltage value of \(V_{F}=150\) mV, was utilized [26]. The generalized configuration approach of the utilized rectifier is illustrated in Figure 3. Two different impedance-matching circuits were designed to interconnect the antenna module with the branches of the rectifier [21]. The system was designed using the Harmonic Balance (HB) method in the Advanced Design System software (ADS-\(\copy\) Keysight Technologies 2000-2021). An FR-4 substrate (dielectric constant \((\epsilon_{r})=4.4\), substrate thickness = 1.6 mm, dielectric loss tangent \((tan)=0.02\), and copper thickness = 0.07 mm) was applied for the design of the utilized system. Furthermore, the Greinacher voltage-doubler topology was selected due to its higher output DC voltage against other alternative topologies for a given input power level [27]. Moreover, [PERSON] et al. [28] reported that the Greinacher voltage multiplier can be employed in high-power WPT systems, while [PERSON] et al. [29] presented an output DC voltage over 420 mV for an input power equal to \(-10\) dBm. In some cases, if we consider low-power values at the input of the rectifier, single-diode topologies may outperform voltage-doubler topologies. However, the single-diode topology was not considered in this work, due to the comparative DC output voltage characteristic of the Greinacher topology. Figure 2: The design process of the proposed Schottky diode-based rectifier. Figure 4 portrays the configuration of the Greinacher voltage-doubler that was utilized in the presented rectifier [21]. The voltage-doubler uses 2 zero-bias Schottky surface-mounted Avago HSS285C series diodes and 2 capacitors \(C_{1}=C_{2}=\)100 pF (AVX 08053K) with 1% tolerance. The rest of the voltage-doubler components are several conductor lines of the appropriate width (W) and length (L) to ensure a compact layout design. The input RF AC signal was rectified by the voltage-doubler circuit. The latter was comprised of a voltage clamp (diode \(D_{1}\) and capacitor \(C_{1}\)) and a peak rectifier (diode \(D_{2}\) and capacitor \(C_{2}\)).