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Understanding AC Power Source Measurements, Part 2: Frequency and Power

Posted by Grady Keeton on Mar 31, 2016 10:23:32 AM

Programmable AC power sources are primarily used to provide a low distortion, precisely controlled sinusoidal voltage to a unit under test, but some AC sources, such as the California Instruments I-iX Series II, perform measurements as well. Part 1 describes the benefits of using sources for measurement and how to make voltage and current measurements. In Part 2, we'll discuss how to make frequency and power measurements.


Frequency is measured at the output of the AC source based on the voltage signal. For AC sources that use a direct digital synthesizer (DDS) to generate the output reference clock, the frequency accuracy of AC source's output often exceeds the accuracy of the frequency measurement.

Real and Apparent Power

Apparent power (VA) can be calculated by multiplying the RMS voltage with the RMS current. Thus, an AC source that does not offer this function but does measure RMS voltage and current can still provide VA readings when used in a test system. The test program can do the calculation.

Measuring true power, however, is not this simple. Unless the load presented by the unit under test (UUT) is purely resistive, the voltage and current will not be in phase, and the true power is less than the apparent power. Only a true power measurement will provide an accurate measurement of the power being dissipated by the UUT.

True power measurement requires the integration of the instantaneous voltage and current product over time. The result of this calculation is shown in the figure below. Notice that the instantaneous power dips below zero for some period of time. Since many UUTa are not purely resistive, true power is an important measurement function.

Screen Shot 2016-03-15 at Tue, Mar 15 - 7.06PM

For more information, download the application note “Understanding AC Power Source Measurements.” You can also contact AMETEK Programmable Power Sales at 858-458-0223 or email the Sales Department at sales.ppd@ametek.com.

Topics: AC Power Sources, Frequency and power

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