13-20 824 Reference Manual 9/29/09
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 its side-band levels. The third
harmonic is found by tripling the fundamental frequency,
then finding the nearest bin frequency and summing its level
with its side-band levels. The fourth and higher harmonics
are found by the process described above by summing the
energy of these 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.
The THD as calculated in the 824
satisfies all of the requirements for
ANSI S3.6.
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
sum calculation will include only the 3
rd
and/or 4
th
bins
but not the 1
st
or 2
nd
bin. In any case the noise energy sum
calculation will never include the first four bins.
• The fundamental must be higher than any of the first four
bins. There must not be excessive DC or near DC levels.
• THD+n must be at least 10 dB below the fundamental
level. There must be an identifiable tone.
• The fundamental frequency will be displayed whether
there is an identifiable tone or not.
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