a signal discriminator for preventing activation by undesired signals of low power transponder circuits in a keyless entry system . these undesired signals may be noise or interfering signals . an asymmetrical low pass filter is used to determine the presence of a desired signal having a defined length of time . the asymmetrical filter has a longer charge time then discharge time , thereby being adapted to quickly discharge upon the loss of a signal . signal duration verification is further used to determine if the desired signal is of a correct time duration .