Fully Automated Electrically Controlled Tunable Broadband Interferometric Dielectric Spectroscopy for Aqueous Solutions

A fully automated electrically controlled tunable interference-based microwave sensor is demonstrated with broadband complex permittivity measurements of aqueous solutions within the frequency range of 12–18 GHz. To construct deep destructive interference over the above-mentioned bandwidth, two quadrature hybrids (QHs or 90° coupler) split and combine the signals of material-under-test (MUT) channel and reference liquid (REF) channel with 180° phase shift. Together with voltage variable phase shifters (PSs) and attenuators in both channels, the destructive interference nulls of the desired frequency can be easily tuned having a
$Q$
-factor of
$2.67\times {10}^{3}$
. A coplanar waveguide (CPW) sensor design was used as the microwave sensing structure and closed microfluidic channel was designed and manufactured with polydimethylsiloxane (PDMS) to facilitate the alteration of MUT, REF, and calibration liquids. Benefiting from electrically controlled PS and attenuator, the measurement system is fully automated for accurate and fast broadband microwave liquid characterization. Measurement of 2-propanol-water solutions (isopropyl alcohol (IPA) solutions) with volume concentration of 1% is performed to illustrate automation processes and its high sensitivity. A mathematical model improved from those in the literature by taking conductor loss and mismatch effects into consideration was applied to extract MUT permittivity, generalizing the calibration procedure with the air and deionized (DI) water as calibration materials, which results in improvements in accuracy, sensitivity, and effective MUT volume. Comparing to state-of-the-art sensors, the proposed setup has an average relative error (ARE) of 1.443% for the effective volume of 5.18
$\mu \text{L}$
. The complex permittivity of 2-propanol-water solutions with different mole fractions were obtained and confirmed with current literature data.