Microwave Active Resonator for High Resolution Sensing Applications

A passive interdigitated capacitor (IDC) resonator with active feedback topology is proposed for sensing applications. The BJT BFP760 transistor from Infineon technology is used to design the active circuitry to improve the performance of the passive IDC sensor in terms of its resolution and quality factor. The proposed active sensor resonates at around 3.5 GHz with an improved quality factor of 12000 as compared to that of the passive structure with a quality factor of around 300. The sensing mechanism for the designed active resonator is demonstrated by first simulating the corresponding passive structure in the CST MWS, and then importing its scattering parameters into ADS to facilitate integration with the active circuitry. The designed structure is then fabricated and measured on a 0.8 mm thick Rogers $\left(\varepsilon_{\mathrm{r}}=2.2\right)$ substrate. The measured response under the OFF (Passive), and ON (Active) state shows the effectiveness of the employed active design topology. The improvement in the sensor resolution in terms of its sensitivity is about 46.7%, while the Q factor improvement is about 40 times as compared to that of the corresponding passive IDC structure.