The diagram below shows a measurement window starting when the sync source rises through
zero and ending at the sync source's last rising zero crossing within the measurement window.
The measurement’s period is the number of cycles that fit into the measurement window
multiplied by the sync source period. The next CWA measurement begins at the end of the
previous measurement. The portion of the waveform after the last zero crossing is thus included
in the following measurement window, ensuring gapless measurement. Therefore, the
periodicity of results is shorter than the measurement window. For example, a 200-ms window
produces data every 192.3 ms for a 52-Hz signal.
The sync source determines the measurement period. The other signals will be measured over
an integer number of cycles if their frequencies match the sync source. This is important for 3-
phase measurements, as all three phases must be measured simultaneously to properly
calculate total power. Furthermore, accurate efficiency analysis requires simultaneous
measurement of input and output power.
This measurement window is used to compute point by point power, RMS voltage and current,
and average (arithmetic mean) voltage and current. These three quantities become inputs for all
other CWA power measurements. The frequency of the signals is measured by counting zero
crossings and dividing the number of cycles by the time used to complete those cycles.
A good sync source is essential. In a system that generates a low frequency AC voltage output by
switching at a high frequency to DC rails, zero crossings may be hard to detect. In this case, you
might synchronize on the current, which will often have much lower frequency content and a
more sinusoidal appearance. Contrarily, consider a rectified circuit with small current pulses at
the voltage peaks and zero current otherwise. In this case, the voltage signal would be the sync
source.
The CWA feature includes a noise reject setting to improve zero crossing detection on noisy sync
sources by changing the hysteresis level applied to zero crossing detection. Higher noise reject
settings lead to higher positive and negative edge thresholds for sync source zero crossing
detection. Therefore, a very small signal, say 1% of range, may not cross these thresholds for a
high noise reject setting, but may cross them at a lower setting. For a very noisy signal, use high
noise rejection unless the signal is small relative to its measurement range.
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Keysight IntegraVision PA2200 Series Power Analyzers Operating and Service Guide
