11/2/00 FFT (Optional) 11-9
THD information is only available
on screen at the time of the measure-
ment and is not stored with a file or
printed to a report.
Results are reported in dB down difference and units of per-
centile. The dB down difference is calculated by subtracting
the harmonic level from the fundamental level. The percent-
age is equal to one hundred times ten raised to the power of
the result of the dB down difference divided by twenty.
Entering the THD display will auto-
position the cursor to the fundamen-
tal frequency on the FFT screen.
THD is calculated by scanning the full spectrum to find the
fundamental frequency and selecting the highest peak. The
actual frequency is extrapolated from the fundamental bin
frequency and the highest sideband bin frequency. The sec-
ond harmonic is found by doubling the fundamental fre-
quency, then finding the nearest bin frequency and summing
its level with it’s side-band levels. The third harmonic is
found by tripling the fundamental frequency, then finding
the nearest bin frequency and summing its level with it’s
side-band levels. The fourth and higher harmonics are found
by the process described above for the second and third har-
monics.
THD+n is calculated by summing the energy of the funda-
mental bin with the five bins on each side, then subtracting
the energy sum of all the other levels.
For THD to be calculated, the following criteria must be
met:
• The window must be in Hanning mode after an overall
reset. If not, “Available only in Hanning Window” is
shown.
• The fundamental must be above the 4
th
bin or 10 Hz
whichever is higher. In the case where the fundamental
frequency is near the 5
th
bin and it’s skirt extends into
some or all of the first four bins, the fundamental energy
THD+n percentile
Harmonic percentiles
4th & higher harmonic
percentile
4th & higher harmonic
Harmonic dB down
THD+n dB down
Fundamental frequency
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