Injection-Locked-Oscillation-Based Active Sensor on Dual-Mode Folded SIW Rectangular Cavity for High-Resolution Detection to Moisture and Fe Particle in Lubricant Oil

The TE201 and TE102 modes of the folded substrate integrated waveguide (FSIW) rectangular cavity (RC) are used as frequency-selective elements to be coupled with a negative-resistance circuit (NRC) to implement a dual-mode negative-resistance (NR)-based voltage-controlled oscillator (VCO). Then, a wideband low-power-level signal from vector network analyzer (VNA) is injected into the NR-based VCO to implement an injection-locked-oscillation (ILO)-based dual-mode FSIW RC active sensor for separately detecting moisture and Fe particle in lubricant oil. With the help of an external static H-field, the lubricant oil passes through the shared sensing region where the strong time-harmonic H-field of TE201 mode and the E-field of TE102 mode are highly concentrated. Then, a LabVIEW-based real-time measurement system for ILO frequency is developed to characterize the moisture content in the volume fraction from 0.025% to 0.5% and Fe particle concentration from 40 to 200 mg/dL. The measured ILO frequency self-shift noise
$\Delta f_{\mathrm {self}}$
of the TE201 (TE102) mode of the active sensor loaded by pure lubricant oil is 15 kHz. The ILO frequency shift
$\Delta f_{s}$
of the dual-mode active sensor loaded by lubricant oil sample is recorded to demonstrate a high sensitivity (2.8 MHz/vol% for moisture and 5.8 kHz/mg
$\cdot $
dL
$^{-1}$
for Fe particle) and low limit of detection (0.025% for moisture and 40 mg/dL for Fe particle).