## 3 Development of RF WPT-Enabled IoT Application
### Implementation and Experimental Validation
#### 3.4.3 RF-DC Converter  
The third key component of the solution is the RF-DC converter. Likewise, for the antenna, there are almost no ready-made commercial RF-DC solutions available today. The only exception to this rule is the P2110B RF energy harvesting chipset [18] from Powercast(r) (Pittsburgh, PA, USA), the cost of which exceeds 30 EUR (34 USD) for one chip [64]. In larger quantities, the cost may reduce, but still stays rather high. Therefore, we developed our own solution, which is discussed in what follows. The first challenge with respect to the RF-DC converter design is the selection of the diodes to be used. The components need to be picked mindful of such parameters as the threshold and reverse-breakdown voltage, element parasities, and harmonics. The former parameter defines the minimum level of the radio signal, from which the circuit could collect the energy (i.e., the \"sensitivity\" of the circuit), and affects the efficiency of the energy harvesting. The junction resistances and capacitances as well as the package inductances and capacitances influence the efficiency and limit the maximum operating frequency. Finally, the cost and the dimensions of the diodes also play a role, since a single rectifier circuit may employ multiple diodes. Second, one has to decide the architecture and the number of stages in the rectifying circuitry. Some of the possibilities are reported and discussed, e.g., in Ref. [11]. Importantly, the number of stages needs to be specified based on the expected range of the input radio signal's power. Specifically, more stages enable collecting the energy from weaker signals, but at the same time introduce some losses which affect the efficiency of the system when harvesting the power from stronger signals. In addition, the more stages increase the dimensions and the price of the system. The former challenge can be addressed by employing a multi-path or switching rectifiers (e.g., the ones proposed in Ref. [65]); nonetheless, this makes the design of a rectifier even more complex and costly. Finally, the rectifier has to be matched with the antenna to prevent the energy reflections to the environment. Such matching is a non-trivial task due to the nonlinearity of the diode's characteristics and namely the variation of the diode's impedance depending on the power and frequency of the input radio signal and needs to account for the specifics of the Printed Circuit Board (PCB) and the manufacturing processes. For this study, we have designed and produced own energy harvesting circuitry shown in Figure 10. The form factor of the PCB is such that it can be connected to our in-house developed modular IoT HW platform [2; 66]. For the board, we used FR4 with 0.8 mm substrate height and 0.18 \(\upmu\)m trace thickness. The substrate dielectric is \(E_{r}=4.8\). With these parameters, 1.4 mm wide line provides 50 Ohms impedance. We applied a Dickson charge pump built using the Avago HSMS 285C pair serial connected diodes (San Jose, CA, USA), which are optimized for low voltage operation. The schematic of the designed RF-DC converter board is illustrated in Figure 11.  
After assembling the PCB, its matching to the antenna was manually tuned. Namely, we used the Anritsu MS4623B vector network analyzer (Atsugi, Kanagawa, Japan) to estimate the input impedance and used it to calculate the matching components. Furthermore, the matching components were fine-tuned to fight the imperfections and tolerances of the components themselves, as well as to address the effects of manual component assembly. The measured reflection coefficients for the designed board are illustrated in Figure 12. We also measured the efficiencies of the designed RF-DC converter board for the different power levels--the respective results are presented in Table 4. As one can see, the performance is in the same range as that of the state-of-the-art works, discussed in Section 2.