Patent ID: 7026803

Claim:
A capacitive amplifier for detecting and amplifying an electrical tone conducted by one of a group of wires in order to identify and trace a particular wire, the capacitive amplifier suppressing noise signals having a predetermined fundamental noise frequency and suppressing noise signals having frequencies that are harmonics of said predetermined fundamental noise frequency, said capacitive amplifier comprising: a probe for being placed adjacent a wire under test; an input terminal, coupled to the conductive probe, for receiving an input signal therefrom; a suppression unit, coupled to the input terminal, for receiving the input signal and for suppressing said noise signals, said suppression unit comprising an analog-to-digital (AD) converter, coupled to the input terminal, for converting the input signal into a digital signal; a memory, coupled to said A/D converter, for storing digital signal values; a subtraction unit for receiving time-delayed digital signal values from said memory and for subtracting time-delayed digital signal values from digital signal values; and a digital-to-analog (D/A) converter coupled to said subtraction unit; and an amplifier, coupled to the suppression unit, to amplify the noiseless output signal; said suppression unit being arranged for receiving the input signal and for providing a time-delayed signal of the input signal and to subtract the time-delayed signal from the input signal, the time-delayed signal being delayed by a delay period substantially equal to the inverse of the noise frequency to be suppressed or an integer multiple thereof, and said subtraction unit being part of a central processing unit (CPU) or its functions being performed by said CPU and said CPU being arranged for reading out digital signal values from said memory after a storage time, said storage time being identical with said delay time, and for subtracting these time-delayed digital signal values from actual input digital signal values.