• Since there are few analog circuits, measurements of high accuracy and reliability from instrument to instrument are more easily obtained.
• The digital format eliminates the divergence in temporal correlation of the measured values that occurs due to differences in the response characteristics of LPFs and other devices. For example, since the power factor is calculated from the measured values of voltage, current, and power, the simultaneity of the measured values is important. Measurement and calculation can occur simultaneously.
• Good accuracy (linearity) is achieved with extremely small inputs. and the power factor error is small.
• Sampled data can be processed digitally, enabling waveform display, analysis, and other functions.

· FIR Type Digital Filtering Provides Rapid Response

With FIR type digital filters, power values are calculated by averaging all data sampled within the effective sampling period. For accurate measurement, the sampling period must be set to one or several times the input period. Therefore the zero level of the input signal (zero cross point) is detected in a comparator circuit, and the effective sampling period that is synchronized with the input period is detected. The power value is obtained by dividing the total sampled data in this effective sampling period by the number of samples N.

With this method, in principle, rapid response can be achieved since active power can be calculated by averaging results from a single period.

· IIR Type Digital Filtering Provides Easily-Obtained Stable Measured Values
With IIR type digital filters, active power is determined by smoothing the calculated instantaneous power. There is no need to detect the period, and theoretically there are no periods of paused measurement. Therefore, these filters have the advantage of being able to obtain stable measurements.

3.5. Harmonic Measurement Function: PLL Circuits and FFT Calculation for Power Measurement and Evaluation of Power Quality
The measurement principle here is the same as that of FFT analyzers. However, in contrast to analysis of frequency criteria by an FFT analyzer, the powermeter’s harmonic analysis function analyzes the frequency orders in the integer multiples of the fundamental wave. Therefore it is necessary to obtain samples that are synchronized to the fundamental frequency. This synchronized sampling is achieved by the PLL circuit. Figure 7 shows an overview of the PLL circuit.

The phase comparator compares the phases of two input clocks and pulse-outputs a phase difference signal. When voltage is applied, the phase difference signal is converted to DC in a loop filter and sent to a voltage controlled oscillator (VCO) that can change the oscillation frequency. The output from the VCO feeds back to the phase comparator. Before arriving at the phase comparator, the frequency output from the VCO is converted to its reciprocal (1/N) to become N times the frequency of the input frequency.
This process allows the samples to be synchronized to the input signal, and facilitates accurate measurement of the input signal’s fundamental wave and its integer multiple components. The equations for the fundamental wave components are shown below.