Low-cost analog signal chain for transmit-receive circuits of passive induction-based resonators

TL;DR

This paper tackles the challenge of building a low-cost, passive TxRx signal chain for inductively driven magneto-mechanical resonators (MMR) and LC resonators. It introduces two circuit variants: Type 1 uses a DPDT EMR relay with a class-D amplifier, and Type 2 employs a full H-bridge to enable pulsed, high-frequency excitation with isolated gate control. Experimental results show that both designs can drive MMRs and capture their decaying response, with the H-bridge offering faster switching and higher excitation capabilities at the expense of coil-impedance losses. The work demonstrates the feasibility of scalable, multi-channel, low-cost isolation schemes for passive sensing and tracking in ultra-low-frequency regimes, and provides practical guidance on frame timing, switching mechanisms, and LNA configurations.

Abstract

Passive wireless sensors are crucial in modern medical and industrial settings to monitor procedures and conditions. We demonstrate a circuit to inductively excite passive resonators and to conduct their decaying signal response to a low noise amplifier. Two design variations of a generic transmit-receive signal chain are proposed, measured, and described in detail for the purpose of facilitating replication. Instrumentation and design aim to be scalable for multi-channel array configurations, using either off-the-shelf class-D audio amplifiers or a custom full H-bridge. Measurements are conducted on miniature magneto-mechanical resonators in the ultra low frequency range to enable sensing and tracking applications of such devices in different environments.

Figures (3)

• Figure 1: Overview of MMR signal chain. The principal components required to drive, sense and locate MMR or LC resonators are shown in (a). The depicted sensor responds to pressure changes in the environment by decreasing the magnet-to-magnet distance, thereby increasing the measured resonant frequency. Evaluation is performed in real-time on a computer, that also controls the amplitude, phase and frequency of the next transmit cycle to continuously pump the oscillator. In (b), an image of the prototype device including class-D amplifier, TxRx-switch and data acquisition system is shown. Different coils, such as a planar $4$-element coil array or the 3D square-shape Helmholtz coil can be connected and exchanged.
• Figure 2: Two types of TxRx switches. An EMR relay with a DPDT configuration is used in type 1 (a). The COM contacts connect to the coil array $L_1$ and a class-D amplifier with a full-bridge output stage. In (b), a different design is proposed named type 2, based on an H-bridge without PWM or filtering, combining amplifier and TxRx switch into one unit.
• Figure 3: MMR measurement results. A single frame for both proposed TxRx switch types is shown, with superimposed low (green) and high (black) amplitude measurement. The sequence cycle of fixed length is divided into $T_\text{tx}=100$ ms (blue) and $T_\text{rx}=1900$ ms (orange). The excitation frequency is $200$ Hz for this specific MMR, consisting out of a spherical and a cylindrical permanent magnet. Frequency, amplitude and phase can be controlled channelwise in real-time between different frames. During excitation, the coil current $i_1$ in $L_1$ is measured and connected to the idle ADC input to provide accurate feedback. Zooms show $5$ Tx and Rx periods of each measurement and the switch window at the beginning of the Rx window.