High resolution microwave microstrip resonator for sensing applications

A microwave planar resonator with a very high quality factor is presented for sensing applications. The proposed resonator uses an active feedback loop with a microwave amplifier to generate negative resistance to compensate the resonator's loss and increases the loaded quality factor of the system. This high quality resonator is based on a planar microstrip resonator with a primary quality factor of 200. The active loop technique increases the primary quality factor up to 15,480 with no other material in its surrounding environment when measured at 1.55 GHz resonance frequency. The high quality factor of the designed resonator provides very high resolution for detection of permittivity variation in its ambient environment with a theoretical resolution of 0.1 ppb (10^-10). It is experimentally demonstrated that the small permittivity variations such as that of foam (with permittivity very close to air) can be detected. The circuit demonstrated a resolution of 26 dB in amplitude for a 1 MHz variation in the frequency domain.