These instruments provide the product of the two input signals (both voltages) through an analog multiplier that evaluates a voltage output proportional to the power of the input signals. This output can be followed by a filtering block.

The output signal is proportional to the instantaneous electric power flowing through the line. To calculate the energy, it is now necessary to complete the process by integrating over the observation time.

This last procedure can be performed in two different ways.

1st Procedure
. The power signal at the output of the analog multiplier is applied to the input of a voltage-frequency converter. Thus, the power information is converted from a voltage level to the frequency of pulse sequence, for which the counting process performs the integration of the power in the observation interval, i.e., the measurement of energy.

The final measurement can be performed by means of an electronic counter with digital display or using a dc step motor incrementing the rotor angular position every pulse by a fixed angular increment. The rotor position is shown by a mechanical counter (similar to the system mounted on the induction energy meters) indicating the total number of complete rotations performed by the system, proportional to the energy of the system under measurement.

This second arrangement is normally adopted because it allows a permanent record of the energy information, which is not subject to possible lack of electric energy as in the first case.

2nd Procedure
. This arrangement is based on an analog-to-digital converter (ADC) connected to the output of the analog multiplier. The sampling process is driven by an internal clock and performed by a sample and hold circuit.

Thus, the ADC provides uniform sampling over the signal period and, under the condition imposed by the sampling theorem, the sum of the samples is proportional to the integral of the power signal, i.e., to the energy during the observation interval.

The calculation is performed by means of a dedicated CPU and then the results are sent to the digital memory to be stored and displayed. They can also be used to manage any other automatic process based on the energy measurement.

For this purpose, data are available on a data bus (serial or parallel) connecting the measuring system with other devices.

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