Patent ID: 7408454

Claim:
A tire information detecting apparatus, comprising: an interrogator which transmits an interrogation signal toward a tire in a vehicle at a transmitting timing, and receives a reply signal including tire information at a receiving timing so as to process the reply signal; a responder which has a sensor being excited by a predetermined frequency signal to detect the tire information and is disposed in the tire to return the reply signal to the interrogator at the receiving timing in responding to the interrogation signal; a mixer unit which receives the interrogation signal having a first signal and a second signal which is spaced away from the first signal by an amount equal to the predetermined frequency, from the interrogator, and outputs a signal of the frequency difference between the first and second signals to the responder by mixing the first and second signals, wherein the interrogator includes a first voltage-controlled oscillator for outputting the first signal, a first PLL circuit to which a first reference signal is input and which controls the first voltage-controlled oscillator, a second voltage-controlled oscillator for outputting the second signal, a second PLL circuit to which a second reference signal is input and which controls the second voltage-controlled oscillator, and wherein the first signal is input to the first PLL circuit, and the signal of the frequency difference between the first and second signals is input to the second PLL circuit, where the signal of the frequency difference between the first and second signals is input to the sensor.