CMOS wireless temperature sensor with integrated radiating element

Abstract

A novel fully integrated wireless temperature sensor realised in 0.35 μm CMOS technology, operating at 2.4 GHz, is presented. The realised chip emits an electromagnetic field at regular time instants, set by an on-board digital control circuit. The high frequency oscillator is a simple three-stag ring oscillator. The signal is transmitted by a small on-chip loop antenna structure. This ring oscillator with the on-chip antenna represent a building block of a wireless temperature sensor, which transforms the silicon substrate temperature variation into a frequency modulation. To easily recover the frequency- vs.-temperature information, the electronic design of the ring oscillator has been carefully tuned to get a linear dependence between them. However, the frequency oscillation of a ring oscillator presents an undesired dependence on the bias voltage. To solve this problem a technique was implemented based on the monitoring of two different signals, allowing the extraction of reliable information on temperature, regardless of the bias voltage variation. This concept is implemented using two three-stag ring oscillators with slightly different frequency- vs.-temperature characteristics, both close however to 2.4 GHz, switched alternatively on for a few milliseconds each. The experimental measurements show that the oscillators provide an electromagnetic field with a power level of −45 dBm at a distance of 1 m. The peak and average power consumptions are 3.3 mW and 500 μW, respectively.