Thermal converter with quartz crystal temperature sensor for ac-dc transfer

The design and features of an experimental thermal ac-dc converter with frequency output are presented. A bulk acoustic wave LC-cut quartz resonator with linear frequency versus temperature characteristic is used as a temperature sensor. The resonator is excited through an inductively-coupled Butler oscillator. The sensitivity of this device is about 1 kHz/spl middot/K/sup -1/. In order to reduce the electromechanical coupling between the heater and the vibrating area, a thin-film Ni-Cr heater was sputtered at a location where the convex surface of the resonator intersects the nodal plane of the vibrations. The overall temperature coefficient of the output frequency is about -0.3%/spl times/K/sup -1/. The responsivity of the device is 300 kHz/spl times/W/sup -1/. The short-term stability of the output frequency permits an ac-dc transfer at the 10/sup -6/ level. The ac-dc voltage transfer difference of the converter is less than 10/spl times/10/sup -6/ at an input voltage of 3 V in the frequency range from (1-5) kHz. Below 1 kHz, the ac-dc transfer difference increases proportionally to the input power and doubles with a decade decrease of the input frequency. No resonances of the quartz plate were detected between 5 kHz and the frequency of the strongest fundamental thickness shear mode (9.5 MHz). Hence the converter can be probably used up to 1 MHz.