The increasing demand for wireless power transfer (WPT) and energy harvesting (EH) systems has driven extensive research in the field of radio frequency (RF)-based WPT and EH, thus facilitating the charging of electronic devices regardless of distance and angle. Rectifiers play an essential role in converting the received RF signals into usable DC power for various applications, including Internet of Things devices, sensor networks, and wearable electronics. Single-band rectifiers are designed to operate optimally at a specific frequency band as determined by the target RF source [1–4]. However, the RF environments for WPT and EH with multiple RF sources operating at different frequencies and distances are subject to fluctuations caused by interference, fading, and the presence of other wireless devices. Hence, relying on a single-band rectifier can be suboptimal and limit the potential of WPT and EH systems. The pursuit of efficient WPT and EH has led to the development of multi-band [5–15] and wideband rectifiers [16–18] as promising avenues for adapting to such dynamic conditions, thus ensuring continuous EH even when one of the frequency bands experiences unfavorable conditions. This is made possible by the diversity of frequency bands that can be tapped into for WPT and EH technologies, encompassing cellular communication bands such as GSM900, GSM1800, 3G, 4G, and 5G as well as ISM radio bands at 0.915 and 2.4 GHz.

Various types of multi-band rectifiers with half-wave transmission lines [6], stepped transmission lines [9–10], and Tsections [11] matching networks have been introduced for high-efficiency operations within limited operating frequency ranges. Although multi-band rectifiers employ various classes of operation, including class-F [14] and inverse class-F [8], [15], to enhance power conversion efficiency (PCE) at multi-frequencies, they include complex circuit configurations. A rectifier array using automatic impedance transformation was introduced to extend the input power range over multi-bands [19]. However, there is a disadvantage in that the number of load resistances and diodes increases. In [20], a dual-band rectifier based on a dual-band matched voltage doubler was proposed to encompass wide input power and frequency ranges. Because the matching networks connected to each diode of the voltage doubler influence one another, numerous iterations are necessary for optimization.

In this paper, a broad dual-band rectifier is proposed for the operating frequency diversity and wide input power dynamic range of WPT and EH systems. A voltage doubler based on shunt capacitors was introduced to achieve similar impedance values over wide frequency bands. An independent dual-band impedance control network (IDICN), which comprises two-stage L-type matching networks based on two shorted stubs at 3.4 GHz, was proposed to independently control the input impedance of a voltage doubler at two frequencies, achieving a high PCE in a simple structure. When the input power ranged from 17 to 21 dBm, it had a peak PCE of over 70% within frequency ranges of 1.8–2.6 and 2.9–3.4 GHz. The proposed rectifier achieved a PCE of over 50% within the wide input power ranges of 14 and 15 dB at 2.45 and 3.4 GHz.

Fig. 1 shows a schematic of the proposed broad dual-band rectifier. It comprises a voltage doubler, an IDICN, a DC pass filter, and a load resistor R_L. The target center frequencies f₁ and f₂ of the proposed rectifier were set to 2.45 and 3.4 GHz, respectively. As shown in Fig. 2, the voltage doubler with the shunt capacitors was configured to have a similar impedance value within a wide frequency and input power range by minimizing the impedance variation of the diode according to the input power and frequency [21]. However, the voltage doubler should be precisely matched to the load at optimal input powers to achieve a high PCE (>70%) within the two targeted frequency ranges.

In this design, we introduced an IDICN to independently control the input impedance Z_VD of the voltage doubler within broad input power and frequency ranges in a simple configuration. The proposed IDICN comprises two λ/4 shorted stubs at 3.4 GHz along with two series transmission lines.

At 3.4 GHz, TL₁ transforms Z_VD at 16 dBm of input power to 50 Ω, as shown in Fig. 3. Because phase θ₂ of the series transmission line TL₂ is set to 90º at 3.4 GHz, input impedance Z_IN2 of the proposed rectifier with TL₂ is the same as that of Z_IN1 at 3.4 GHz. Then, Z_IN1 at 16 dBm of input power at 2.45 GHz can be transformed to approximately 50 Ω. Similarly, the width of the shorted stub TL₃ and the width and length of the series transmission line TL₄ can be adjusted to match Z_IN2 with the source impedance. Because phase θ₃ of TL₃ is 90º at 3.4 GHz, only the series transmission lines are utilized to move Z_IN1 to the load impedance at 3.4 GHz. By properly optimizing parameters Z₁, Z₂, Z₃, Z₄, θ₁, and θ₃, Z_IN can be matched to 50 Ω within the wide input power ranges at two frequencies.

As shown in Fig. 3, transmission lines TL₁ and TL₂ in the ICIDN transform Z_VD at 16 dBm of input power to approximately 50 Ω at two frequencies simultaneously.

As shown in Fig. 3, transmission lines TL₃ and TL₄ in the ICIDN are optimized to position Z_IN in the voltage standing wave ratio (VSWR) = 2 circle, thus ensuring high PCE within the input power ranges of 3–23 and −1.5–22 dBm at 2.45 and 3.4 GHz, respectively. Fig. 4 illustrates the simulated input impedance of the proposed dual-band rectifier at different frequencies and input power of 16 dBm. It was confirmed that the input impedance Z_VD of the matched voltage doubler was located near 100–j57 within a wide frequency range of 2.0–3.8 GHz. The proposed IDICN can move Z_VD in the VSWR = 2 circle within the frequency range.

The return loss |S₁₁| at 16 dBm for different frequencies is shown in Fig. 5. It indicates that the proposed IDICN assures |S₁₁| is less than −10 dB for the frequency range of 2.0–3.8 GHz. Fig. 6 shows the simulated PCE of the proposed dual-band rectifier with different input powers and frequencies. The proposed rectifier ensures especially high PCEs at 2.45 and 3.4 GHz. The PCE remained over 50% within a wide input power range of 3.5–18 dBm when the operating frequency varied from 1.8 to 3.9 GHz. Furthermore, the frequency ranges of the PCE >70% were 2.0–2.6 and 2.9–3.5 GHz. Although the input impedance of the proposed rectifier was matched to the load over a wide frequency range, a specific load resistance was chosen to achieve maximum PCE at 2.45 and 3.4 GHz, leading to efficiency variations with frequency. Fig. 7 shows the simulated PCE with different load resistances at 16 dBm of input power. The implemented rectifier achieves maximum PCE at 2.45 and 3.4 GHz when it has a load resistance in the range of 800–900 Ω.

The proposed dual-band rectifier at 2.45 and 3.4 GHz was designed and fabricated on a TLC-32 substrate with a thickness of 0.79 mm, a relative dielectric constant of 3.2, and a loss tangent of 0.003. Fig. 8 illustrates the layout and shows a photograph of the fabricated rectifier with dimensions of 44.2 mm × 35.4 mm. The voltage doubler structure was implemented using the HSMS-286C Schottky diode. The DC pass filter consisted of a 10 nH inductor and two parallel capacitors, each with a capacitance of 10 pF. As shown in Fig. 9, the RF incident power P_IN within the range of −3–25 dBm was generated using the E8247C signal generator, 83020A power amplifier, and IG002.004S. 17 isolator. An optimal R_L of 820 Ω was selected to obtain the maximum PCE of the proposed rectifier at both 2.45 and 3.4 GHz. The output voltage V_DC was measured using a digital multimeter, and the PCE was calculated as V_DC2/(R_L·P_IN).

Fig. 10 illustrates the measured PCE of the proposed rectifier with different input powers. The proposed rectifier obtained peak PCEs of 74.7% and 74.4% at 2.45 and 3.4 GHz, respectively. The PCEs were higher than 50% within an input power range of 7.9–21.9 dBm and 6.9–21.9 dBm at these two frequencies, respectively. The difference between simulation and measurement occurred owing to the fabrication tolerance and inaccuracy of the diode model according to the input power [15]. Fig. 11 depicts the simulated PCE at 16 dBm and the measured PCE at 20 dBm. It is observed that both the simulated and measured PCEs are maintained above 70% within the broad dual bands of 1.8–2.6 and 2.9–3.4 GHz.

Table 1 presents a comparison between the rectifiers proposed in this study and other rectifiers from recently presented studies. As observed, the proposed rectifier exhibits the highest peak PCE at the two frequencies and operates on a broader frequency band than most other rectifiers. Furthermore, the input power range with PCE >50% of the proposed rectifier has a wider range than any of the rectifiers presented in previous studies.

In this study, a broad dual-band rectifier operating in the 2.45 and 3.4 GHz bands was proposed for multi-band wireless power transfer systems. The proposed IDICN with two shorted stubs having a quarter wavelength at 3.4 GHz can cancel out the capacitive imaginary part of the input impedance seen from the voltage doubler and independently transform the high real part of the input impedance to 50 Ω at two frequencies. The implemented rectifier obtained peak PCEs of 74.7% and 74.4% at 2.45 and 3.4 GHz, respectively. It maintained a PCE of over 50% within the wide input power ranges of 7.9–21.9 dBm and 6.9–21.9 dBm at 2.45 and 3.4 GHz, respectively. Furthermore, the voltage doubler configuration with shunt capacitors assures comparable input impedance across the wide frequency range of 2.0–3.8 GHz. With the IDICN, achieving Zin in the VSWR = 2 within the frequency range enables a peak PCE of more than 70% within the frequency ranges of 1.8–2.6 GHz and 2.9–3.4 GHz.