Fuente: WIPO "meat" An electronic transponder implanted under the skin of an animal to be identified or located receives from an interrogator a VHF signal at frequency f1 which via band pass filter 10, rectifier 12, and lowpass filter 14 closes time switch C to power up section D of the transponder from local battery E for five seconds. The interrogator follows the first signal with a digitally coded VHF signal at frequency f2 (150 MHz). The coded signal is decoded and compared with a stored code in decoder 18 which on a positive comparison acts to close time switch F so that the transponder transmits an uncoded VHF return signal at frequency f3 for 34 seconds from oscillator G via aerial A to be received by the receiver of the interrogator. Since the interrogator ''knows'' the code of the interrogating signal sent, then if it receives a return signal it knows that a transponder (and animal) labelled with that code is present. The time switches conserve battery power for an animal's lifetime, and the VHF frequencies allow interrogating ranges of up to 40 metres...