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This paper presents two new wireless temperature sensors made with a standard CMOS process exhibiting an on-chip antenna. The realized chips include a 3-stage ring oscillator structure which transforms the silicon substrate temperature variation into a frequency modulation. The first device works at a frequency of about 750 MHz @ 30degC. The signal is transmitted by a small loop antenna structure which is realized by aluminium deposition on the top surface of the chip. 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 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 mum process, improving the antenna efficiency with respect the first one.