CMOS fully-integrated wireless temperature sensors with on-chip antenna

Two novel wireless temperature sensors made with a standard CMOS process exhibiting an on-chip antenna are presented. The realized chips include a 3-stage ring oscillator structure which transforms the silicon substrate temperature variation into a frequency modulation. The signal is transmitted by a small loop antenna structure which is realized on the top surface of the chip. A first chip was realized, with the aim of demonstrating the feasibility of such an antenna structure. The device works at a frequency of about 750 MHz @ 30°C. The electronic design of the ring oscillator has been carefully tuned to get a linear dependence of frequency vs. temperature. However, a bias voltage dependence is observed. An improvement of this design involves the implementation of a technique, based on a mathematical procedure between two different signals, that allows the extraction of reliable information on temperature, regardless of the bias voltage variation, and without recurring to power consuming voltage regulators. This concept is implemented, on the same chip, by using two 3-stage ring oscillators with slightly different frequency-vs-temperature characteristics, both close however to 2.4 GHz, switched alternatively on for a few milliseconds. Each ring-oscillator has its own antenna and the device is realized with a standard 0.35 μm process. The higher frequency highly improves the antenna efficiency, which have now a three-fold larger area with respect to the previous case.