Abstract
An ultrasmall-capacitance-variation detector was developed that consists of a VHF-band LC oscillator and sensing pads. 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 up to the attofarad level. A superheterodyne system is added to the LC oscillator to convert the VHF signal to 2 MHz range. A low power CMOS device, which requires 20 mW range, can be used to handle the 2 MHz signal, while high power CPLD such as 300 mW is required for the direct count of VHF signal. This conversion system saves 280 mW dissipation and contributes to achieve the battery operation of the sensor. As an application of the sensing system, a wireless sensor for the non-invasive detection of pests inside wood was demonstrated.
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