A ZERO-Power Sensor Using Multi-Port Direct-Conversion Sensing

This paper presents a class of zero-power microwave sensor architecture based on the direct-conversion principle to eliminate data processing at the Internet of Things sensors and provide unpowered nodes. A base station (BS) transmits a single tone signal at the frequency of f
₀
/2 toward the sensing node using an antenna. At the node, an antenna receives the signal, and a passive frequency doubler makes the frequency twice. Then, a multi-port structure directly modulates the sensing data at the frequency of f
₀
and sent back to the BS by an antenna. The multi-port circuit has one input, one output, and some loading ports. In this paper, a six-port modulator and four similar sensitive capacitive resonators are used. A pair of resonators senses the variation of a sample under test (SUT) while the other pair is covered by a reference or known material. At the BS, any quadrature receiver can be used to demodulate sensing data. Here, a similar six-port structure is used to extract data and find the SUT variations. An example one-node system is implemented at f
₀
= 2.45 GHz and evaluated by some standard SUTs. To support multiple nodes, a smart directional antenna is necessary at the BS, which also improves the overall efficiency of the system.