Patent ID: 7421891

Claim:
A tire monitor radio circuit performing radio transmission to a tire pressure detector mounted on a wheel of a vehicle, the tire monitor radio circuit comprising: a control circuit delivering a binary digital baseband signal; a modulation circuit having an oscillation circuit generating a carrier wave, the modulation circuit switching between an oscillatory state where the carrier wave is delivered and a non-oscillatory state where the carrier wave is not delivered, based on inversion of a binary value of the digital baseband signal; an antenna resonance circuit which has a coil and a capacitor both connected together and is capable of resonating in response to the carrier wave delivered by the modulation circuit; a resistance damping circuit which has a switching element and a resistance and is connected to the antenna resonance circuit, the resistance damping circuit being switchable between an operative state where a resonance current of the antenna resonance circuit can be applied to the resistance and an inoperative state where the resonance current of the antenna resonance circuit cannot be applied to the resistance; and a compensation circuit putting the resistance damping circuit into the operative state when the modulation circuit is switched from the oscillatory state to the non-oscillatory state, the compensation circuit returning the resistance damping circuit to the non-operative state when or before the modulation circuit is switched from the non-oscillatory state to the oscillatory state.