Quad-Band Rectifier Circuit Design for IoT Applications

Abstract

In this work, a novel quad-band rectifier circuit is introduced for RF energy harvesting and Internet of Things (IoT) applications. The proposed rectifier operates in the Wi-Fi frequency band and can supply low-power sensors and systems used in IoT services. The circuit operates at $2.4$, $3.5$, 5, and $5.8$ GHz. The proposed RF-to-DC rectifier is designed based on Delon theory and Greinacher topology on an RT/Duroid 5880 substrate. The results show that our proposed circuit can harvest RF energy from the environment, providing maximum power conversion efficiency (PCE) greater than 81% when the output load is $0.511$ kΩ and the input power is 12 dBm. In this work, we provide a comprehensive design framework for an affordable RF-to-DC rectifier. Our circuit performs better than similar designs in the literature. This rectifier could be integrated into an IoT node to harvest RF energy, thereby proving a green energy source. The IoT node can operate at various frequencies.

1. Introduction

The Internet of Things (IoT) can make the world more green, efficient, and connected. The need for a wireless and autonomous battery management system has become more urgent due to the number of devices used per household. The answer to this situation is power harvesting that allows a circuit to operate on its own energy from the environment.

Physical devices in an IoT network are connected through numerous wireless networks and systems. The different nodes and devices connected to IoT systems are noticeably high. It is estimated that there will be more than 30 billion IoT-connected devices by 2025, accounting for almost four per person on average. Thus, this imposes an impressive challenge to self-sustainability [1].

The electromagnetic energy that exists in the environment can resolve energy issues in next-generation systems. Radio frequency energy harvesting (RFEH) could be a viable solution to meet the energy demands of wireless systems, such as portable devices that need to be recharged several times a day. Research on wireless power transmission (WPT) and RF energy harvesting in the millimeter and microwave ranges has been a dynamic area of research in recent years [2]. This technique can power many devices wirelessly and can also prove useful for diagnostic and therapeutic implantable devices [3], robotics [4], wireless sensor networks, wireless charging, and military devices [5].

The use of batteries is challenging for many economic, environmental, and practical reasons. Alternative energy sources can complement or even replace batteries [6]. Several methods for scavenging energy have been proposed. Nevertheless, a system without batteries for energy harvesting to convert RF to DC power is much preferred. This approach provides a consistent, clean, and green energy solution for portable low-power systems.

The WPT is divided into two main categories: far-field microwave power transmission and near-field magnetic inductive powering [7,8]. Far-field microwave power transmission provides power delivery at a long distance, while near-field power transmission is restricted by a short transmission distance. With the growth in communication technology, it is possible to scavenge the radio frequency (RF) signals of the environment through microwave power transmission. Various types of transmitters, such as radio towers, mobile phones, televisions, Wi-Fi routers, and base stations, can continuously charge numerous low-power devices [9].

A rectenna system (rectifying antenna) is composed of an antenna, a rectifier circuit, an IMN (impedance matching network), and a DC–DC converter [10,11]. The antenna collects the electromagnetic waves from the environment and drives them to the rectifier circuit. The rectifier converts ambient RF signals to DC power. The DC–DC converter boosts the rectifier’s output. The IMN has to provide the desired impedance matching between the receiving antenna and the rectifier with minimal energy losses. Figure 1 shows a typical RF energy harvesting system.

There are several frequency bands in the environment that are distributed over a wide range. Hence, many rectennas work in different frequency bands. The RF range is part of the electromagnetic range that begins from VHF (very high frequencies) to EHF (extremely high frequencies), ranging from 3 kHz to 300 GHz. Prominent transmission systems including FM radio (87.5–108 MHz), VHF and UHF DTV, GSM-900, GSM-1800, UMTS-3G, LTE-4G, Wi-Fi, Bluetooth, GHz, and 5G utilize the RF range for broadcasting and communication purposes [12].

In RFEH, the main challenge faced is the low RF power density available in the environment. To evaluate the possible locations of RF energy harvesters, the level of RF power must be measured in the sensor environment. Hence, due to the multitude of sensor applications, these measurements must take place in various environments, such as cities, villages, and the countryside [13,14]. The solution to this problem is the design of broadband or multi-band rectenna systems to collect RF energy from different frequency bands to increase power conversion efficiency (PCE) [15].

The rectifier is the key part of the rectenna system that affects the efficiency of the whole system [16]. The design of the rectifiers has to be very compact. Rectifiers can transfer energy with minimal losses to other parts of the rectenna. In IoT networks, there is a huge number of applications for rectifiers. For example, in [17,18,19,20,21], there are applications in which a rectifier is vitally important. Usually, rectenna systems are manufactured to operate at frequencies that are important for wireless communications and broadcast services.

In this work, we designed a quad-band rectifier that works in the Wi-Fi band [22] and, more specifically, at $2.4$, $3.5$, 5, and $5.8$ GHz. The RF power density of a GSM base station is higher than the signal from a Wi-Fi router. The distance of a Wi-Fi router is very small (indoor environments); thus, higher power levels can be harvested [23].

The remainder of this work is organized as follows. Firstly, we present the materials and methods in Section 2. In Section 3, we present the design framework. In Section 4, we present the complete design of the proposed quad-band rectifier and the numerical results. In Section 5, we discuss the contributions of this work and its main results. Finally, Section 6 summarizes the concluding remarks of our work.

2. Materials and Methods

2.1. Related Work

Rectifiers, which are responsible for transforming RF signals into DC power, can be broadly classified into integrated designs such as CMOS and monolithic microwave integrated circuits (MMICs), as well as discrete designs utilizing diodes or transistors. Numerous noteworthy studies in the bibliography investigate these design techniques [24,25,26,27,28,29,30].

In [1], the authors designed a multiband rectenna for RF energy harvesting. The proposed rectifier circuit achieves 78% PCE. The authors in [9] manufactured a rectenna system that achieves a maximum power conversion efficiency of 60% at $2.45$ GHz and 53% at $3.5$ GHz, for input power 0 dBm. In [15], we can find a dual-band rectifier circuit design that achieves 53% power conversion efficiency at $3.5$ and 5 GHz, for input power 12 dBm. The circuit was designed on a Rogers RT/Duroid 5880 substrate. The authors of [31] designed a rectifier circuit with a PCE of $51.3$%, a frequency of $5.1$ GHz, and an output load of 350 $\mathrm{\Omega }$. In [32], the designers manufactured a bridge rectifier that operates at 5 GHz. The circuit’s PCE is up to $36.4$%.

In [33], the authors manufactured a full-wave (FW) rectifier that operates at sub-6 GHz in 5G bands with a central frequency of $3.5$ GHz. This rectifier achieves a maximum power conversion efficiency equal to $42.5$% when the output load is $1.1$ kΩ and the input power reaches 9 dBm. The authors in [34] designed, at a frequency of $3.5$ GHz, a Villard voltage doubler that reaches 42% PCE with 14 dBm input power. In [35], the authors presented a full-wave rectifier that achieved $39.6$% power conversion efficiency at $3.8$ GHz using an HSMS-2820 Schottky diode. In [36], the authors designed a rectenna that can scavenge energy from WiMAX and Wi-Fi bands. This half-wave circuit (HW) is on a Rogers RT/Duroid 5880 substrate and operates at $3.5$ GHz with a 0 dBm input with an efficiency of 44%. A full-wave rectifier was manufactured in [37] with a PCE of $29.7$% for an input power of 6 dBm at $3.5$ GHz.

An HW rectifier can be found in [38]. This circuit using the SMS-7630 diode achieves a PCE of $42.5$%, for input power equal to $-10$ dBm. In [39], we can find a rectenna system that works at $2.4$, 4, and 5 GHz. The authors in [40] designed a triple-band rectenna that achieves 53%, 31%, and $15.56$% power conversion efficiency at 2, $2.5$, and 3 GHz, respectively. The authors in [41] designed a full-wave rectifier on a Rogers RO3003 substrate with 0 dBm as input power and two Schottky diodes, SMS7630-079LF. They reached a power conversion efficiency of 42% at $3.5$ GHz. In [42], the authors, using SMS7630 diodes, manufactured a rectifier that works with GSM900, GSM1800, UMTS2100, WiMax 3500, and Wi-Fi ($2.4$ and 5 GHz). They achieved $33.5$% maximum PCE at $-15$ dBm when the frequency was $1.8$ GHz.

A dual-band rectifier was manufactured in [43], which achieves $49.9$% PCE with $1.1$ kΩ load resistance and 0 dBm input power at 5 GHz. The authors in [44] fabricated a rectenna circuit for the frequency range of $1.8$, $2.4$, $5.8$, and $3.8$ GHz. They achieved PCE equal to $62.69$% for input power 0 dBm. The rectenna proposed in [45] achieves maximum PCE equal to $69.3$% at $2.4$ GHz. In [46], we can find a rectifier circuit design that works at $2.45$ GHz. They achieve maximum PCE 57% for input power $-5$ dBm. The authors in [47] designed a bridge rectifier. The maximum power efficiency is equal to 80% at $-10$ dBm input power.

In [48], we can find a rectifier circuit that was designed to operate at the $5.8$ GHz Wi-Fi frequency band using SMS7630-061 Schottky diodes. In [49], we can find a half-wave rectifier that achieved $63.5$% efficiency for 0 dBm input power. The authors in [50] presented a rectifier circuit that operates at $5.8$ GHz using HSMS-286B diodes. In [51], the authors fabricated a full-wave rectifier, which achieves 60% power conversion efficiency. In [52], the authors manufactured a high efficiency RF–DC converter and achieved a high output voltage of $34.2$ V.

2.2. Design Methodology

In this work, we present a quad-band rectifier circuit that operates in the whole Wi-Fi range at frequencies $2.4$, $3.5$, 5, and $5.8$ GHz. The rectifier circuit design procedure consists of four main steps: defining the proper substrate, choosing the circuit topology, selecting the appropriate diode, and designing an efficient IMN [10]. The design of each part must be well-developed to ensure that the whole system develops in a feasible way [16].

The decision regarding the appropriate substrate is a key part of the design procedure. The ideal substrate for a rectifier circuit would be a substrate with a low dielectric constant and low dielectric loss. The selection of the circuit topology is the main thing to consider before selecting the individual elements of the circuit. The choice of the appropriate diode is another very important step in the procedure, since we want a diode that gives us a high conversion efficiency because of its characteristics. After that, we must select the rest of the elements for the rectifier circuit (caps, stubs, etc.). The next step is to calculate the frequency response ($S_{11}$). We adjust the circuit elements with the antenna impedance (impedance matching network) and recalculate the $S_{11}$. If we reach the desired values for frequency, $S_{11}$, and power, then we calculate the PCE of the overall system. Otherwise, we return to the previous calculations [15]. Figure 2 displays the methodology.

2.3. Contribution

The main contribution of this work lies in the following:

• We introduce a novel quad-band rectifier circuit that operates in the whole Wi-Fi range.
• We discuss and provide a complete design framework for a low-cost RF-to-DC rectifier.
• We propose a novel IMN (impedance matching network) that comprises two discrete branches, one for $2.4$ and $3.5$ GHz, and the other for 5 and $5.8$ GHz. Each branch consists of a combination L and reverse L-shaped network with rectangular and radial stubs.
• We design a rectifier circuit that operates at $2.4$, $3.5$, 5, and $5.8$ GHz simultaneously.
• Our rectifier design performs better than similar designs in the literature.
• This rectifier could be part of an IoT node to harvest environmental RF energy, therefore providing a green energy source. The IoT node can operate at different frequencies.

3. Design Framework

3.1. Receiving Antenna

The key role of the receiving antenna in a rectenna system is to collect as many RF signals as possible. The reflected power at the antenna input, the efficiency of the antenna, the impedance, the gain, and its weight and size are the basic factors that are considered in the design of an antenna. Among the most common antenna types are the log-periodic dipole arrays [53], the monopole antennas [54], the dipole antennas [55], the fractal antennas [56], the Yagi–Uda antennas [57] the bowtie antennas [58], the loop antennas [59], the dielectric resonator antennas (DRAs) [60], and the planar inverted F antennas [61]. Furthermore, the microstrip antenna [62] is one of the common designs found in the literature for RF energy harvesting.

3.2. Topology of the Rectifier

The design theory of a rectifier circuit looks mature. Circuit designers often need mathematical simulators to design RF rectifier circuits. They must search for many topologies and find the best load resistance or input matching impedance. RF rectifiers are expected to maximize the RF energy they receive as much as possible. This requirement is very important for energy harvesting and wireless power transfer applications [63]. The main topologies for a rectifier design are the diode bridge, the single diode, and the voltage multiplier. We selected a Greinacher voltage multiplier for our circuit because this topology converts and amplifies the AC input to the DC output [5,64]. The circuit consists of two diodes (D1, D2) and two capacitors (C1, C2). In this circuit, a half-wave of input sinusoidal voltage surges through one diode. Then, the opposite half-wave passes through the other. Repeating the process amplifies the voltage to double and charges the final capacitor. Hence, in the output, there is double voltage [65]. Figure 3 displays the Greinacher voltage multiplier.

We classify the ground as GND, the AC input source as AC, the output load as $R_L$, and the input and output voltage as $V_{in}$ and $V_{out}$, respectively.

Our rectifier is based on the Delon voltage doubler topology. Jules Delon, a French engineer, presented his topology in 1908 in Marseilles. The idea is to combine two HW rectifier circuits on top of each other. The load in this circuit is related to the differential path. The voltage in the output load is the difference between the two outputs [66]. Our rectifier has two branches, and each branch consists of a Greinacher topology. Figure 4 shows our selection for the rectifier topology.

3.3. Substrate

The choice of the substrate is a key part of the design procedure for rectifier circuits. The low dielectric loss and the low dielectric constant are the most important characteristics. Hence, for our design, we select the substrate RT/Duroid 5880. It has a dielectric loss tangent of $0.0009$, a thickness of $0.508$ mm, a copper thickness of $0.035$ mm, and a dielectric constant of $2.2$ [67]. These characteristics mean that this substrate is appropriate for high-frequency applications, such as the proposed quad-band rectifier that works at $2.4$, $3.5$, 5, and $5.8$ GHz. Figure 5 shows what the substrate looks like.

3.4. Diode

Diodes are a key component in rectifier circuit design. Usually, in the environment, there are only low power levels. The magnitude of the incident signal can be equal to the $V_{TH}$ (voltage threshold) of the diode. So, the diode losses become predominant [68].

The conversion efficiency of the rectifier is related to the diodes. Equations (1) and (2) below give us the PCE of the circuit:

$$P_m=\sum _{i=1}^n{P}_{fi},$$

(1)

$$n_m={\displaystyle \frac{P_m-P_{LOSS}}{P_m}},$$

(2)

where $P_{LOSS}$, $n_m$, $P_m$, n, $P_{fi}$, are the power loss related to the diode, the power conversion efficiency of the rectifier, the input power of the rectifier circuit, the number of operating bands, and the power of each operating band, respectively [1].

In rectifier circuit designs, the most commonly used are Schottky diodes because they have fast switching capabilities and detect very low signals [69,70]. For this voltage multiplier, we selected the HSMS-286C Schottky diodes [71]. These diodes have been manufactured and optimized to operate at frequencies between 915 MHz and $5.8$ GHz. According to the manufacturer, these diodes are ideal for detector circuits and voltage doublers [72]. We achieved a high conversion efficiency because of their features.

We can see these features below:

• Barrier capacitance $C_{J0}=0.18\mathrm{pF}$;
• Series resistance $R_S=6\mathrm{\Omega }$;
• Breakdown voltage $B_V=7\mathrm{V}$;
• Typical capacitance $C_T=0.25\mathrm{pF}$;
• Forward Voltage $V_F=250–350\mathrm{mV}$.

3.5. Impedance Matching Network

In the literature, there are numerous types of impedance matching networks. These networks consist of distributed elements such as microstrip lines and stubs or lumped elements such as inductors and capacitors. The design of these networks is the main difference among the plethora of rectifier designs in the literature. These networks ensure that there will be power transfer with minimal losses from the receiving antenna to the rectifier circuit. In addition, this prevents any irrelevant RF energy from reaching the rectifier from the receiving antenna, affecting the efficiency of the rectifier in the main band of interest [73,74]. This network guarantees a minimum power reflection of the signal back to the source [75].

The proposed rectifier operates at $2.4$, $3.5$, 5, and $5.8$ GHz; hence, we chose distributed elements because they are appropriate at frequencies between 3 and 300 GHz. At frequencies below 3 GHz, the lumped elements are generally used [10].

The proposed impedance matching network consists of two different branches: one for $2.4$ and $3.5$ GHz and the other for 5 and $5.8$ GHz. Each branch consists of a combination of L and reverse L-shaped networks with rectangular and radial stubs. In addition to regular stubs, we select radial stubs because they require less space on the chip. Moreover, regular stubs are better at lower frequencies, such as at $2.4$ GHz [1,76]. Figure 6 illustrates the proposed IMN circuit. In

Table 1. Physical parameters.

Parameter	Width/Length	Angle
TL1	2/30 mm	-
TL2	2/13 mm	-
TL3	2/3 mm	-
TL4	2/$9.5$ mm	-
TL5	2/$7.5$ mm	-
TL6	1/$4.5$ mm	-
TL7	2/13 mm	-
TL8	2/$7.5$ mm	-
TL9	2/$2.5$ mm	-
TL10	2/14 mm	-
TL11	1/$1.5$ mm	-
TL12	$4.2$/$9.3$ mm	-
STUB1	200 nm/$5.2$ mm	$61.5$ degrees
STUB2	3/5 mm	$69.7$ degrees
STUB3	200 nm/6 mm	$20.8$ degrees

, we can find the physical parameters of the transmission lines and stubs of our impedance matching network.

3.6. DC–DC Converter

A high-gain DC–DC converter is applied as an interface circuit between the rectifier and the load to boost and stabilize the output of the rectifier [15]. This type of DC–DC converter has to achieve a high voltage conversion ratio and should have small dimensions. The growth of high-voltage gain converters has emerged as one of the most important additions in the design of renewable energy systems, and more specifically in RF energy harvesting systems where the power level is too low. In the literature, numerous DC–DC converters can achieve high-voltage conversion ratios. Renewable energy applications and IoT systems generally use isolated or non-isolated classes of high step-up DC–DC structures [77,78].

4. Numerical Results

4.1. Proposed RF-to-DC Rectifier Circuit Results

We introduce the novel design of a quad-band rectifier circuit design on the RT/Duroid 5880 substrate. The presented impedance matching network is a network with two distinct branches: one for $2.4$ and $3.5$ GHz and one for 5 and $5.8$ GHz. As mentioned in Section 3, each branch consists of a combination of L and reverse L-shaped networks with rectangular and radial stubs. We designed and then connected two full-wave Greinacher voltage multipliers, one for each branch, based on the Delon theory.

The voltage doublers were built with HSMS-286C Schottky diodes and two sc-avx-ACCU-F-08053K-F-19960828 with 100 pF capacitors. In addition, the circuit encloses several conductor lines of suitable width (W) and length (L) to connect all components (capacitors, diodes). The output load is connected differentially. Consequently, we created a differential rectifier circuit design to achieve higher output voltage and power at low input power [79]. We used the gradient optimizer algorithm from the commercial software Advanced Design System (ADS) from Keysight Technologies to design and optimize this rectifier circuit. Figure 7 displays the complete circuit design of the proposed quad-band rectifier. The quad-band impedance matching network (IMN) described in the previous section is used here. The input signal is received by an antenna connected to the feed line via an SMA (SubMiniature version A) connector. Each branch of the network is designed to operate at two different frequencies. The input signals choose the optimal path, either through one branch or through both, depending on their frequency.

As input for designing the rectifier circuit, we used an antenna port of $Z_A=50\mathrm{\Omega }$, while as an output load, our option is a resistance of 511 Ω. The initial impedance of the rectifier circuit is 18.70 − j1.66 at 2.4 GHz, 3.19 − j 27.00 at 3.5 GHz, 52.46 + j14.45 at 5 GHz, and 15.53 + j28.46 at 5.8 GHz. The goal of the proposed impedance matching network is to match the rectifier circuit to the impedance of the antenna (antenna port). We calculated the impedance of the rectifier to be 52.62 − j0.16 at 2.4 GHz, 49.31 − j4.28 at 3.49 GHz, 47.84 − j1.25 at 4.99 GHz, and 50.04 + j0.86 at 5.8 GHz. Figure 8 depicts the frequency response of the reflection coefficient ($S_{11}$). The $S_{11}$ value at $2.4$, $3.49$, $4.99$, and $5.8$ GHz is $-31.85$, $-27.22$, $-31.86$, and $-41.34$ dB, respectively. In addition to that, the same figure displays the bandwidth (BW) ($S_{11}$ < $-10$). This quad-band rectifier is equal to 20 MHz at $2.4$ GHz, 60 MHz at $3.49$ GHz, 80 MHz at $4.99$ GHz, and 59 MHz at $5.8$ GHz.

The selection of the output load is one of the most critical parts of the design process because it determines the total RF-to-DC efficiency n% of the rectifier circuit. Equations (3) and (4) express the power efficiency n%, and we can see that it decreases when the output load increases. The impedance of the energy harvesting circuit is dependent on a variety of factors, including input power level, operating frequency, load impedance, and rectifier or voltage multiplier topologies [80]. When the load impedance is properly matched to the circuit, the maximum power transfer can be achieved. Due to this, there is an optimal load impedance for each input power level and operating frequency. The maximum efficiency of the RF–DC power conversion was obtained exclusively at specific load resistances, as illustrated in Figure 9. The input power levels were distinct. The RF–DC power conversion efficiency is significantly reduced when the load resistance value is either excessively high or excessively low.

We can see that the best output load is 511 $\mathrm{\Omega }$, and after that, the PCE decreases. There are some points where we have a small increase in PCE, after the value of 900 $\mathrm{\Omega }$, because the figure depicts PCE when the circuit works on all operating frequencies simultaneously.

$$n={\displaystyle \frac{P_{out}}{P_{in}}},$$

(3)

$$P_{out}={\displaystyle \frac{{V_{out}}^2}{R_L}},$$

(4)

where:

• $P_{in}$ is the RF input power;
• $P_{out}$ is the output power;
• $V_{out}$ is the output voltage;
• $R_L$ is the output load.

The commercial software of the Advanced Design System (ADS) provides a harmonic balance (HB) simulation to check the efficiency n% and the voltage output $V_{out}$ of our rectifier. Figure 9 depicts the PCE versus the output load $R_L$ of the rectifier circuit presented. We see that the initial design (at frequencies $2.4$ GHz, $3.5$ GHz, 5 GHz, and $5.8$ GHz) with an output load of 511 $\mathrm{\Omega }$ and an input power of 12 dBm achieves a PCE of $81.75$%, which is the maximum conversion efficiency of the proposed rectifier. It is important to note that all frequency bands were excited simultaneously with the same power.

Furthermore, the proposed quad-band rectifier operates very well at low input power. The rectifier achieves a power conversion efficiency of $54.16$% for input power 3 dBm and an output load of 511 $\mathrm{\Omega }$; $41.49$% for input power 0 dBm and output load of 511 $\mathrm{\Omega }$; and 18% for input power $-3$ dBm and output load of $1.1$ kΩ.

In Figure 10, we can see the PCE (power conversion efficiency) versus $P_{in}$ (input power) for each frequency band distinctly ($2.4$ GHz, $3.5$ GHz, 5 GHz, and $5.8$ GHz). We note that the proposed circuit with an output load of 511 $\mathrm{\Omega }$ and input power of 18 dBm reaches PCE $17.12$% at $2.4$ GHz, $23.96$% at $3.5$ GHz, $28.73$% at 5 GHz, and $37.85$% at $5.8$ GHz.

Figure 11 shows the output voltage of the proposed circuit versus the input power for an output load of 511 $\mathrm{\Omega }$. The output voltage is another important value that characterizes the rectifier circuit. Equation (5) gives the output voltage of the circuit:

$$V_{OUT}=V_{OUT1}-V_{OUT2}.$$

(5)

We observe that the larger output voltage is equal to 4.68 V for the input power 18 dBm. As the input power increases, so does the output voltage. Figure 11 shows that an input power larger than 6 dBm can give us an output voltage larger than 1 V. It should be noted that the power of 6 dBm is impossible or very difficult to obtain in practical Wi-Fi applications. So, this is a theoretical analysis to show the capabilities of the circuit from a purely electronic point of view.

Figure 12 shows the layout of the proposed quad-band rectifier circuit.

4.2. Performance Evaluation

Table 2. Comparative results between the proposed quad-band rectifier and related work.

Reference	Type of Circuit (+)	Substrate	Diode	Frequency (GHz)	${\mathit{P}}_{\mathit{in}}$ (dBm)	${\mathit{R}}_{\mathit{L}}$ (kΩ)	PCE (%) @ Desired Frequencies	${\mathit{V}}_{\mathit{out}}$
[1]	Full-wave rectifier +	FR-4	HSMS-2820C	$0.9$, $1.8$, $2.5$, $3.5$, $5.5$, $7.35$	-	-	78	8 (@3.5 GHz) V
[9]	Full-wave rectifier	FR-4	HSMS-2852	$2.45$, $3.50$	0	$1.20$	$68.40$ (@2.45 GHz), $62.20$ (@2.45 GHz)	$1.22$ V
[15]	Full-wave rectifier	RT/Duroid 5880	HSMS-286C	$3.5$, 5	12	$1.74$	53	$3.815$ V
[31]	Full-wave rectifier +	-	-	$5.1$	15	$0.350$	$51.3$	-
[32]	Full-wave rectifier +	-	Gallium Arsenide printed diodes	5	-	-	$36.4$	-
[33]	Full-wave rectifier	RT/Duroid 5880	HSMS-286C	$3.5$	9	$1.1$	$42.5$	-
[34]	Full-wave rectifier +	FR-4	HSMS-286C	$1.9$$2.5$, $3.6$	14	3	42	$5.53$ V
[35]	Full-wave rectifier +	FR-4	HSMS-2820	$2.45$, $3.8$	20	$0.230$	$64.3$	$4.23$ V
[36]	Half-wave rectifier +	RT/Duroid 5880	SMS-7630	$3.5$, $5.8$	0	$0.5$	44	656.88 mV
[37]	Full-wave rectifier	FR-4	HSMS-2860	$3.5$	6	1	$29.7$	$2.80$ V
[38]	Half-wave rectifier	FR-4	SMS-7630	$2.4$, $3.5$	0	-	$61.6$	-
[39]	Full-wave rectifier +	FR-4	NXP BAP50-03	$2.4$, 4, 5	-	-	54 (@$2.4$ GHz)	298 (@2.4 GHz) mV
[40]	Full-wave rectifier	FR-4	HSMS-285C	2, $2.5$, $3.5$	$-2$	$1.1$	61	-
[41]	Full-wave rectifier	Rogers RO3003	SMS-7630-079 LF	$3.51$	0	2	42	-
[42]	Full-wave rectifier	-	SMS-7630	$0.9$, $1.8$, $2.1$, $2.4$, 5	$-15$	-	$33.5$ (@ $1.8$ GHz)	$0.5$ V
[43]	Full-wave rectifier	FR-4	HSMS-286B	$2.45$, 5	0	$0.70$	$68.83$	$0.236$ V
[44]	Full-wave rectifier +	RT/Duroid 5880	SMS-7630	$0.84$, $1.29$, $1.68$, $3.08$, $3.45$, $4.31$, $5.11$, $5.49$	0	5	$45.69$ (@ $3.45$ GHz)	-
[45]	Full-wave rectifier +	RO4003C	SMS-7630	$2.4$	$5.5$	2	$69.3$	-
[46]	Full-wave rectifier	RO4350B	SMS-7630, HSMS-2850	$2.4$	$-5$	3	57	-
[47]	Full-wave rectifier	-	HSMS-2850	$2.4$	$-10$	$1.4$	80	-
[48]	Full-wave rectifier +	-	SMS-7630-061	$5.8$	10	-	-	3 V
[49]	Half-wave rectifier	FR-4	HSMS-2860	$3.5$	0	2	$63.5$	$2.50$ V
[50]	-	-	HSMS-286B	$5.8$	9	$1.3$	72	-
[51]	Full-wave rectifier	-	HSMS-285C	$5.8$	$3.6$–$10.5$	$1.7$	60	$3.5$ V
[52]	Full-wave rectifier +	-	MA4E1319-1	$5.8$	21	4	$73.1$	$34.2$ V
This Work	Full-wave rectifier	RT/Duroid 5880	HSMS-286C	$2.4$, $3.5$, 5, $5.8$	12	$0.511$	$81.75$	$4.68$ V (@18 dBm)

⁺ Results of fabricated circuit.

contains the relative measured results of the proposed rectifier circuit versus related circuits from the literature. The significant parameters that are nominated for the comparison are the type of circuit, the substrate of the circuit, the selected diodes, the frequencies of operation, the input power, the output load, the maximum power conversion efficiency, and the output voltage. From the results of the table, we can conclude that our rectifier circuit performs better than similar designs in the literature. It is designed on an RT/Duroid 5880 substrate and presents a fine-tuning operation in the frequency bands of interest. Furthermore, it has very good values at the tuning frequencies, and it adopts an impedance matching network technique with a relatively high complexity. Lastly, our rectifier circuit achieves satisfactory power conversion efficiency and high DC output voltage, which makes it a very good circuit for RF energy harvesting in Internet of Things (IoT) applications.

5. Discussion

During the last few years, the Internet of Things has attracted the attention of many researchers. A bibliographic search using the keywords “Internet of Things” and “RF Energy Harvesting” in the Scopus database shows that there are 180,169 articles related to the Internet of Things (IoT) and 5074 articles related to RF energy harvesting in the last 15 years.

The Internet of Things contains many physical devices connected through several networks and wireless systems. The use of batteries is challenging for many economic and environmental reasons. Alternative energy sources can complement or even better replace batteries. The RF energy that exists in the environment can resolve the energy issues in the wireless systems of an IoT network. This approach provides a consistent, clean, and green energy solution [6].

A rectenna is the main and appropriate system in RF energy harvesting. It comprises 4 subsystems, an antenna, a rectifier circuit, an impedance matching network, and a DC–DC converter [10,11]. These systems are designed to operate at frequencies that are important for wireless communications and broadcast services. The radio frequency range is part of the electromagnetic range that begins from VHF to EHF; the Wi-Fi band is also part of this range [12]. We designed a quad-band rectifier that works in the whole Wi-Fi band and, more specifically, at $2.4,3.5,5$, and $5.8$ GHz. The main challenge we faced was the low RF power density that is available in the environment.

The role of the receiving antenna in a rectenna system is to collect as many RF signals as possible [10].

The rectifiers that convert RF signals into DC power can generally be categorized into integrated designs and discrete designs that use diodes or transistors [5,64]. We introduce a novel quad-band rectifier circuit that operates in the whole Wi-Fi range designed with distributed elements. This rectifier could be part of an IoT node to harvest environmental RF energy. The design procedure for the rectifier circuit comprises four main steps, which we presented and followed.

The proposed circuit is a Greinacher voltage multiplier based on the Delon voltage doubler topology [66]. This topology converts and amplifies AC input to DC output. The design was made on an RT/Duroid 5880 substrate [67] using HSMS-286C Schottky diodes [68]. The impedance matching network has two branches, one for $2.4$ and $3.5$ GHz and the other for 5 and $5.8$ GHz. Each branch consists of a combination L and reverse L-shaped network with rectangular and radial stubs.

A high-gain DC–DC converter is applied as an interface circuit between the rectifier and the load to boost and stabilize the output of the rectifier [77,78].

To design and optimize the rectifier circuit, we used the gradient optimizer algorithm from the commercial software Advanced Design System (ADS) from Keysight Technologies. We found the impedance of the rectifier to be $52.62-i0.16$, $49.31-i4.28$, $47.84-i1.25$, and $50.04+i0.86$ at the four frequencies of operation ($2.4,3.5,5,5.8$ GHz). The $S_{11}$ is $-31.85$, $-27.22$, $-31.86$, and $-41.34$ dB at the same frequencies, respectively. The operation bandwidth is 20 MHz at $2.4$ GHz, 60 MHz at $3.49$ GHz, 80 MHz at $4.99$ GHz, and 59 MHz at $5.8$ GHz. The initial design power conversion efficiency is $81.75$%, while the maximum output voltage is equal to $4.68$ V. These results make this circuit a very good option for RF energy harvesting in Internet of Things (IoT) applications.

We introduced a novel quad-band rectifier featuring a new impedance matching network (IMN) and presented a comprehensive design framework. Our rectifier circuit operates simultaneously at four frequencies and outperforms similar designs. However, the circuit design has some limitations that could pose challenges for further development and practical application. Firstly, the circuit performs optimally at an input power of 12 dBm. Secondly, the matching circuit is complex, making adjustments difficult if it does not align well with the antenna. Additionally, it is important to note that our circuit outputs voltages exceeding 1 V when the input power is above 6 dBm. While this is promising from a theoretical electronic perspective, achieving 6 dBm in a typical Wi-Fi environment is challenging for wireless transmission.

In the future, our goal is to improve the power conversion efficiency and output voltage of the rectifier circuit. We also plan to fabricate the design for comparative analysis and integrate it into a larger project. Furthermore, we intend to develop an antenna to create a complete rectenna system.

6. Conclusions

In this work, we presented a novel quad-band RF-to-DC rectifier at $2.4,3.5,5,$ and $5.8$ GHz. The rectifier was designed on an RT/Duroid 5880 substrate. This circuit is suitable for working within the Wi-Fi band. Avago HSMS-286C diodes were used, while the Greinacher topology based on the Delon theory was selected. The circuit presents a PCE equal to $81.75$% and an output DC voltage equal to $4.68$ V when the output load is 511 $\mathrm{\Omega }$, which can supply low-power electronics systems in IoT applications.