9/29/09 FFT (Optional) 12-9
THD (Total Harmonic Distortion)
The THD display shows the results of the THD calculations.
THD information is only available
on screen at the time of the
measurement and is not stored with a
file or printed to a report.
Results are reported in dB down difference and units of
percentile. The dB down difference is calculated by
subtracting the harmonic level from the fundamental level.
The percentage 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
fundamental 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
second harmonic is found by doubling the fundamental
frequency, 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 harmonics.
THD+n is calculated by summing the energy of the
fundamental 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:
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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