2 Making Measurements80 U1461A/U1453A User’s GuideFrequency measurement techniques– Measuring the frequency of a signal helps detect the presence of harmoniccurrents in neutral conductors and determines whether these neutral currentsare the result of unbalanced phases or non-linear loads.– Frequency is the number of cycles a signal completes each second. Frequencyis defined as 1/Period. Period is defined as the time between the middlethreshold crossings of two consecutive, like-polarity edges, as shown inFigure 2-15.– The tester measures the frequency of a voltage or current signal by countingthe number of times the signal crosses a threshold level within a specifiedperiod of time.– If a reading shows as 0 Hz or is unstable, the input signal may be below or nearthe trigger level. You can usually correct these problems by manually selectinga lower input range, which increases the sensitivity of the tester.– If a reading seems to be a multiple of what you expect, the input signal may bedistorted. Distortion can cause multiple triggerings of the frequency counter.Selecting a higher voltage range might solve this problem by decreasing thesensitivity of the tester. In general, the lowest frequency displayed is thecorrect one.– The frequency of the input signal is shown in the primary display. The voltageor ampere value of the signal is shown in the secondary display. The bar graphdoes not indicate frequency but indicates the voltage or ampere value of theinput signal.Figure 2-15 Definition of frequencyRise Time Fall Time+ Width – WidthPeriod90%50%10%
