Patent ID: 7224751

Claim:
Device for checking whether a signal with a predetermined frequency is being received, wherein: (a) the received signal or a signal based on the received signal is compared with one or more threshold values by means of a comparison unit, (b) selected changes or all changes in the result of the comparison are counted by means of a counting unit, (c) the number of changes in the result of the comparison determined is compared with one or more predetermined values, (d) depending on the result of this comparison, a decision unit decides whether a signal with the predetermined frequency is being received or not, (e) a control device is provided which, in the case where a signal with a frequency which is much higher than the predetermined frequency is apparently being received, makes changes in the device by means of which the noise signal level and/or its influence on the check to be performed are reduced, and (f) the counting device overflows when the comparison device is supplied with a signal with a noise signal level exceeding the threshold values of the comparison device.