Active RF tag with wake-up circuit to prolong battery life

An active RF transponder is provided with a wake-up circuit that wakes the RF transponder from a sleep state upon detection of an RF interrogating signal. The active RF transponder includes a battery, an antenna adapted to receive RF signals from an interrogator, and electronic circuitry providing the various RF transponder functions of sending/receiving signals and storing data. A first embodiment of the invention includes a wake-up circuit that periodically checks for the presence of an RF signal at the antenna. The wake-up circuit is coupled to the antenna and includes a switch adapted to selectively couple the battery to the electronic circuitry and provide electrical power thereto upon detection of the RF signals by the antenna. The wake-up circuit further comprises an oscillator providing a clock signal having a low duty cycle that defines intervals during which the antenna is sampled for presence of the RF signals (e.g., approximately 20 ns every 100 mus). A second embodiment of the RF transponder includes a wake-up circuit as in the first embodiment that is further adapted to detect a code sequence modulated in the RF signals. The code sequence is unique for a class of RF transponder, so the wake-up circuit can discriminate between interrogating signals. A third embodiment of the RF transponder includes a wake-up circuit that wakes the RF transponder upon detection of an RF signal that contains data within a desired band of frequencies. This embodiment enables the RF transponder to discriminate between RF signals that likely contain valid data and other RF noise. After the RF transponder has been waked, the wake-up circuit returns the RF transponder to a sleep state if valid data is not detected within a predetermined period of time.