From chemical monitoring to motion detection, sensors provide critical data in a variety of applications, but that data sometimes lacks the specificity to allow decision makers to act.  PNNL’s regenerative feedback resonant circuit can be used to create multi-modal sensors that can pinpoint specific actions or changes in the area being monitored. The system comprises a microwave circuit with a resonant component that is sensitive to changes (e.g., movement, increases in the amount of some chemical, increases in temperature) within the positive feedback path of an amplifier. Filters may also be used in the circuit to select desired operating modes, and the feedback signal is directly sampled using a directional coupler without affecting the circuit response. Potential applications include proximity sensing, through-wall motion detection, radio frequency (RF) emission sensing, magnetometers, acoustic monitoring, and chemical sensing.

The positive feedback enables a very high sensitivity and more precise frequency selection, with measured dynamic quality factors as large as 60 million. The circuit can be designed to accommodate resonator elements with a variety of electromagnetic materials or radiation characteristics and operate at frequencies ranging from UHF to X-band or above. The device has been demonstrated at a wide range of frequencies using multiple prototype circuits.

Sensor prototypes that use an antenna resonator in the feedback path can be operated as a short-range radar system for area monitoring and proximity detection. In this application, multiple sensors can be used to control a desired area and wirelessly transmit any indications of human movement. The system can also detect certain types of RF emissions in the monitored area.

Alternatively, sensors using non-radiating resonator elements combined with electromagnetic materials can be made very sensitive to acoustic signals, optical light, magnetic fields, or temperature. Example resonators include split ring resonators, dielectric ring resonators, and split post dielectric resonators. The rapid response time and continuous monitoring provided by the resonant feedback circuit is well-suited for characterizing a variety of time-dependent phenomena.