An Active Differential Microwave Sensor With Enhanced Anti-Interference Capability for Analyzing Complex Permittivity of Liquid Samples

An active differential microwave sensor with enhanced anti-interference capability for analyzing complex permittivity of liquid samples is proposed in this article. The proposed microwave sensor system is constituted by a differential microwave microstrip sensor and a low-noise amplifier (LNA). The differential microstrip sensor is composed of a splitter, a combiner, and a pair of double-planar circular spiral resonators (DPCSRs), wherein, each DPCSR is etched under each branch of the splitter/combiner. In the experiment, the RF signal produced by the vector network analyzer (VNA) enters into the input port of the microstrip sensor, and exports from the output port of the microstrip sensor. As the insertion loss of splitter/combiner, the magnitude of the output signal is lower than that of the input signal, i.e., <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}_{{21}}\lt {0}~\text {dB}$ </tex-math></inline-formula>. As known, the small signal would be easily affected by the noise signal, which would cause a detection error. In order to diminish the interference and reduce the error, the LNA is cascaded to the output port of the microstrip sensor, and the transmission coefficient (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}_{{21}}$ </tex-math></inline-formula>) of the system is significantly enhanced to be about 6.84 dB. In addition, one branch of the microstrip sensor is regarded as a reference, and another branch is used as a test for the differential structure, which can decrease a certain error. The average sensitivity of the proposed active microwave sensor, in measurement, is about 0.698%, and its maximum detection errors in testing <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varepsilon _{r}^{\prime }$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varepsilon _{r}^{\prime \prime }$ </tex-math></inline-formula> amount to approximately 6.144% and 3.105%, respectively. Generally speaking, the proposed active differential microwave sensor is a good candidate in the field of characterizing liquid samples.