| 摘要 |
A variable frequency radio receiver (1) for a remote keyless entry system for a motor vehicle outputs a valid type data signal through a gate circuit (16) to a signal analysing circuit (3) for further analysing to determine if the data signal insludes a unique code specific to an electronic control unit (2) which controls an alarm circuit, a vehicle immobiliser and a central locking system of the motor vehicle. The radio receiver (1) comprises a micro-controller (4) which operates a variable frequency voltage controlled oscillator (8) for outputting a staircase variable frenquency signal to a mixer (9) where it is mixed with a received signal, and the mixer (9) outputs the received signal at an intermediate frequency which is filtered by an intermadiate frequency filter (10). The signal is then passed on a line (11) through an attenuating circuit (12) comprising five resistors (R1 to R5) which are selectively connectable by switches (SW1 to SW5) between the line (11) and ground, for selectively attenuating the signal for removing noise therefrom. The attenuated signal is fed to an intermediate frequency amplifier (14) and in turn to a demodulating circuit (15) outputs the data signal to the gate circuit (16). The micro.controller (4) analyses the data signal from the demodulating circuit (15) and selects the appropriate value of resistance from the resistors R1 to R5 to be connected between the line (11) and ground to provide the appropriate level of attenuation for attenuating the signal so that the noise level in the signal has little or no effect on the data.
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