What’s Your Take on Chips? Chip Impedance Measurement Practice for RF Energy Harvesting

As our society and economy continue to digitize rapidly, the number of wireless nodes in every square kilometer has become staggering. Experts predict that there could be as many as 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> devices in a single square kilometer, posing significant environmental and economic sustainability challenges. Fortunately, wireless power technologies offer a solution <xref ref-type="bibr" rid="ref1" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[1]</xref>, <xref ref-type="bibr" rid="ref2" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[2]</xref>, <xref ref-type="bibr" rid="ref3" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[3]</xref>. These technologies provide a promising method for wirelessly delivering power to nodes, eliminating the need for batteries and reducing the detrimental ecological impact of batteries, resulting in more cost-effective solutions. <p xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">The battery-less operation of wireless nodes is driven by RF energy harvesters. An RF energy harvester collects incoming RF energy and converts it to dc energy. <xref ref-type="fig" rid="fig1">Figure 1</xref> shows the conceptual schematic of an RF energy harvester. The RF energy harvester typically includes an antenna, an optional matching network, an ultralow-power (on-chip) RF-dc converter, and an output load. For sustainable digitalization, single-chip solutions based on low-cost complementary metal-oxide-semiconductor (CMOS) technologies are superior in cost and power.¹ Previous research on these devices operating at the industrial, scientific, and medical (ISM) frequency bands showed that for an ultralow-power design and battery-less wireless nodes, the power matching of the antenna and the harvester chip becomes mandatory <xref ref-type="bibr" rid="ref6">[6]</xref>, comparable to ultrahigh-frequency (UHF) RF identification (RFID) transponders (tags). Using an additional matching network, as illustrated in <xref ref-type="fig" rid="fig1">Figure 1</xref>, is not possible due to the impracticality of on-chip matching components in terms of chip size and cost. Similarly, off-chip matching components are also not viable because they result in additional power losses or bulky designs. <p xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Power matching of the antenna and the harvester chip means that the antenna and chip impedances are complex conjugate to each other <xref ref-type="bibr" rid="ref7">[7]</xref>. Consequently, the power available in the chip’s circuitry reaches its maximum. The antenna is typically designed to realize power matching <xref ref-type="bibr" rid="ref8">[8]</xref>, <xref ref-type="bibr" rid="ref9">[9]</xref> as the RF chip front end and all its subsystems determine the chip input impedance. Thus, the knowledge of the chip impedance is mandatory for a power-matched antenna design. <p xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">This work reviews and compares various integrated RF energy harvester CMOS chips operating mainly in the sub-1-GHz ISM frequency bands. For a convenient chip impedance analysis, the work provides an overview of common measurement methods by discussing exemplary setups for measuring the impedances of RF energy harvesting chips to enable efficient power matching with their antennas.