# Implementation of an SS-Compensated LC-Thermistor Topology for Passive Wireless Temperature Sensing

## TL;DR

A new passive wireless temperature sensor uses an SS-compensated LC-thermistor setup to track temperature changes through impedance measurements.

## Contribution

The SS-compensated LC-thermistor topology enables direct reflection of thermistor resistance into input impedance for wireless temperature sensing.

## Key findings

- The input resistance at split resonance frequencies matches the thermistor resistance, enabling accurate wireless temperature monitoring.
- Using an average readout provides immunity to small capacitor drift, ensuring stable temperature readings.
- Benchtop experiments with a one-port VNA confirmed the sensor's performance within a tested temperature range.

## Abstract

What are the main findings?

A passive wireless temperature sensor is implemented using SS-compensated magnetically coupled LC tanks.

The thermistor’s resistance is directly reflected in the input impedance at split resonance frequencies.

What is the implication of the main finding?

Enables accurate, battery-free, and contactless temperature sensing through impedance monitoring.

Suitable for applications requiring simple and reliable wireless sensing topologies.

This paper presents a passive wireless temperature sensor based on an SS-compensated LC-thermistor topology. The system consists of two magnetically coupled LC tanks—each composed of a coil and a series capacitor—forming a series–series (SS) compensation network. The secondary side includes a negative temperature coefficient (NTC) thermistor connected in series with its coil and capacitor, acting as a temperature-dependent load. Magnetically coupled resonant systems exhibit different coupling regimes: weak, critical, and strong. When operating in the strongly coupled regime, the original resonance splits into two distinct frequencies—a phenomenon known as bifurcation. At these split resonance frequencies, the load impedance on the secondary side is reflected as pure resistance at the primary side. In the SS topology, this reflected resistance is equal to the thermistor resistance, enabling precise wireless sensing. The advantage of the SS-compensated configuration lies in its ability to map changes in the thermistor’s resistance directly to the input impedance seen by the reader circuit. As a result, the sensor can wirelessly monitor temperature variations by simply tracking the input impedance at split resonance points. We experimentally validate this property on a benchtop prototype using a one-port VNA measurement, demonstrating that the input resistance at both split frequencies closely matches the expected thermistor resistance, with the observed agreement influenced by the parasitic effects of RF components within the tested temperature range. We also demonstrate that using the average readout provides first-order immunity to small capacitor drift, yielding stable readings.