Electronic circuit for measuring electrical energy

Claim 1

A metering apparatus, comprising: current input means for providing a signal that is proportional to the time derivative of an alternating current in a circuit; first clock means for providing a first series of clock pulses, said first series of clock pulses having a generally constant frequency; integrating means for modulating said series of clock pulses with said signal that is proportional to said time derivative of said current, said integrating means being connected to said current input means and said first clock means, the output of said integrating means being a first pulse width modulated signal; means for delaying said first pulse modulated signal by a predetermined amount of time, the output of said delaying means being a second pulse width modulated signal; second clock means for providing a second series of clock pulses, said second clock means having an output connected to said delaying means, said second series of clock pulses having a variable frequency, said second clock means being configured to vary said variable frequency according to a preselected relationship with the frequency of said alternating current in said circuit;