Attofarad-level capacitance variation detector uses RF-sensor with 98/100 MHz oscillator/local superheterodyne scheme for wireless pest sensor

An ultrasmall-capacitance-variation detector was developed that consists of a VHF-band LC oscillator, sensing pads, and a superheterodyne scheme that magnifies the resonant-frequency shift. An oscillator frequency (∼100 MHz) in the VHF band allows a small base capacitance to be used for resonance, which provides a high sensitivity to small changes in capacitance. An RF sensor with a superheterodyne scheme magnifies the frequency shift and enhances the sensor sensitivity up to the attofarad level. The sensitive sensor must be isolated from human movement and other environmental influences to ensure accurate measurements. A simple shield consisting of a small ground loop with a perimeter of 17 cm was added to the sensor assembly. As an application of the sensing system, a wireless sensor for the non-invasive detection of pests inside wood was constructed. It was able to detect the movement of insects inside a wooden post.