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CWA Measurements - Theory and Operation
Continuous Whole-Cycle Analysis (CWA) is a gapless measurement technique. This section
covers theory, configuration, and troubleshooting, and the following section covers the
mathematics of CWA measurements.
Theory of Operation
Oscilloscopes traditionally measure signals based on what is shown on the screen. These
measurements often have gaps, meaning data between measurements that do not get
measured. Furthermore, the accuracy of such measurements is determined by what is on the
screen. Users often want guaranteed accuracy specifications, and having specifications change
based on the horizontal scale is inconvenient for power analysis. To avoid these limitations, the
IntegraVision PA2200 Series power analyzers include a new type of data analysis called
Continuous Whole-Cycle Analysis (CWA).
Only gapless measurements guarantee accurate power consumption measurement, and some
IEC standards require continuous, gapless power measurement. For an AC signal, this analysis
must be performed over an integer number of waveform cycles. In many cases, such as
efficiency measurements or 3-phase power analysis, the user must analyze multiple signals over
a consistent time period. These different signals are often out of phase, so it is helpful to use one
signal to determine the time frame of the analysis. In CWA, the user selects one signal as the
synchronization (sync) source, and the CWA measurement is performed on all signals
simultaneously, based on the sync source's zero crossings. The time frame starts and ends on
rising zero crossings, so that measurements are always performed over an integer number of
sync source cycles. Note that the CWA feature includes a Sync Offset parameter to vertically
offset the zero crossing for sync sources that contain a DC offset.
The CWA feature uses a measurement window to specify how many sync
source cycles are included in each measurement. Longer windows reduce
noise, but miss brief signal events. Shorter windows reveal brief events,
but increase measurement noise and limit the lowest frequency signal that
can be analyzed (a full cycle must fit in the window). Some measurements
use a fixed measurement window, and others allow you to select a window
duration.
The actual window will be slightly longer than the nominal duration so that the instrument
always measures the same number of cycles per window at typical power frequencies, such as
50 and 60 Hz. For example, suppose you have a nominal 50-Hz signal that is sometimes 49.99
Hz and sometimes 50.01 Hz. A 200-ms window would fit 10 whole cycles of a 50.01-Hz signal,
but only 9 whole cycles of a 49.99-Hz signal. Therefore, the 200-ms window is made slightly
longer than 200 ms to accommodate 10 whole cycles of frequencies slightly lower than 50 Hz.
Keysight IntegraVision PA2200 Series Power Analyzers Operating and Service Guide 109
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