Method for designing a temperature measurement system using two phase-locked loops

A method using two phase-locked loops (PLLs) for a temperature measurement system (TMS) is described. In the system, a time-domain temperature sensor can convert the temperature into the duty cycle. The PLL circuit developed to emulate the Vernier caliper to measure the duty cycle is able to eliminate the measuring error and obtain higher resolution without increasing the clock frequency. Then, a single-chip microprocessor is designed to get and compute the duty cycle. Thus, the temperature can be easily computed with the duty cycle, and then sent to a liquid crystal display (LCD) to display. The experimental results show that the resolution of the duty cycle is 1/65280, and the range of the measured temperature is from −25.5 to 102 °C with maximum error ±0.05 °C in the TMS. Therefore, the main advantages of this system are high resolution, high accuracy, and low cost.