Oscillation-Based Active Sensor on SIW Reentrant Cavity Resonator for Self-Sustained Ultralow Concentration Detection to Saline Solution

A back-to-back reentrant cavity resonator (BTB RECR) in substrate integrated waveguide (SIW) configuration is integrated with a negative resistance oscillation (NRO) circuit to implement an SIW BTB RECR NRO-based active sensor for self-sustained sensing application. The dependence of sensitivity on the polarization between the dielectric sample and the induced
${E}$
-field of the passive SIW BTB RECR is investigated to obtain sensitivity as high as possible. A multitask LabVIEW-based real-time test system, which can avoid disassembly and ensure the reliability of the measured results, is built up to selectively measure for transmission coefficient
${S} _{21}$
and frequency spectrum through a microwave switch. During the process of real-time replacement of deionized (DI) water reference solution with NaCl solution in dielectric sensing channel, the in-between real-time recorded oscillation frequency is cleverly used as the frequency-modulated (FM) input signal to the delay-line-based frequency demodulator and is transferred to in-phase
${V} _{{\text {I}}}$
and quadrature
${V} _{{\text {Q}}}$
dc voltages. Then, the modulated
${V} _{{\text {I}}}$
and
${V} _{{\text {Q}}}$
dc voltages are postprocessed into phase angle by using arctangent algorithm, and the phase angle in stable convergence state is finally used as response parameter for self-sustaining measurement for the concentration of NaCl solution. The NRO-based active sensor demonstrates a phase angle self-shift noise of 7.8° and a high-resolution detection to NaCl solution at low concentration level from 2.5 to 80 mM/L and high concentration level from 100 to 800 mM/L. The measured minimum detectable lower limit concentration is 2.5 mM/L, and maximum sensitivity is 40 kHz/mM
$\cdot \text{L}^{-1}$
(0.31°/mM
$\cdot \text{L}^{-1}$
).