Adjustable Low-Cost Highly Sensitive Microwave Oscillator Sensor for Liquid Level Detection

TL;DR

This work addresses the need for high-sensitivity, low-cost liquid level sensing by introducing a microwave oscillator sensor with an adjustable input impedance implemented through a Z2 branch. The approach leverages a negative-resistance oscillator with a tunable input network, a shorted lambda/2 resonator for sensing, and a secondary lambda/4 path to stabilize operation, achieving stable oscillation around a high microwave frequency. Key results include a nonlinearity below ~2.7%, LoD under 0.05 mm, and a sensitivity around 21 kHz per micrometer, validated across multiple liquids and temperatures via CST/ADS simulations and experimental measurements. The sensor demonstrates strong linearity, repeatability, and compatibility with CMOS/MEMS technologies, offering a practical solution for integrated wireless sensor networks and small-scale liquid detection where high dielectric liquids are involved.

Abstract

This paper explores the implementation of a low-cost high-precision microwave oscillator sensor with an adjustable input resistance to enhance its limit of detection (LoD). To achieve this, we introduce a \textit{Z$_{2}$} branch in the input network, comprising a transmission line, a capacitor (\textit{C$_{B}$}) and a resistor (\textit{R$_{V}$}). The sensor is tested with eight different liquids with different dielectric constants, including water, IV fluid, milk, ethanol, acetone, petrol, olive oil, and Vaseline. By fine-tuning the \textit{Z$_{2}$} branch, a clear relationship is found between $\varepsilon_{r}$ of materials and R$_{V}$.Our experimental results demonstrate outstanding characteristics, including remarkable linearity (nonlinearity < 2.44\%), high accuracy with an average sensitivity of 21 kHz/$μ$m, and an excellent limit of detection (LoD < 0.05 mm). The sensor also exhibits good stability across a range of liquid temperatures and shows robust and repeatable behavior. Considering the strong absorption of microwave energy in liquids with high dielectric constants, this oscillator sensor is a superior choice over capacitive sensors for such applications. We validate the performance of the oscillator sensor using water as a representative liquid. Additionally, we substantiate the sensor's improvement through both experimental results and theoretical analysis. Its advantages, including affordability, compatibility with CMOS and MEMS technologies, and ease of fabrication, make it an excellent choice for small-scale liquid detection applications.

Figures (15)

• Figure 1: (a)Combining the oscillator sensor with a FSK transmitter/receiver system b16. (b)Topology of the input and output networks to build the oscillator sensor.
• Figure 2: Geometry of the microstrip transmission line (MSTL) used.
• Figure 3: (a) Small-signal equivalent circuit, (b) ac schematic and (c) simplified ac circuit of the oscillator sensor.
• Figure 4: 3D full-wave electromagnetic model of the sensor resonator in CST Studio Suite and detailed specifications of its dimensions.
• Figure 5: Simulation results using harmonic balance simulator in ADS of Keysight by changing R$_{V}$ from 200 $\Omega$ to 50 $\Omega$:(a) time domain and (b) frequency domain.