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Changes with Amplitude
It is fairly common for the THD+N reading to apparently increase as the
fundamental tone amplitude decreases, even if the device does not create more
distortion or noise at lower amplitudes.
This effect is due to the fact that the THD+N reading is the ratio of the amplitude
before filtering to the amplitude after filtering. If the amplitude after filtering is
dominated by noise, hum, or interference, it will be essentially constant regardless of
the amplitude of the fundamental tone. Therefore, as the amplitude of the
fundamental tone decreases there will be an apparent increase in the calculated ratio.
Since all devices create some noise, any device has a ‘noise floor’. This is the level
ofnoise that is always created by the device. Ifthe fundamental tone has a lower
amplitude than the noise floor, there will be essentially no difference between the
level of filtered and unfiltered signals and the THD+N will be 100%.
In some devices, the unwanted signals may actually increase as signal amplitude
decreases. Ifyou suspect this, you may want to make an absolute THD+N
measurement, which does not include the amplitude before filtering in the
measurement. This is accomplished by selecting any unit besides % or dB.
All devices also have a maximum allowable amplitude. Ifthe fundamental tone is
above this amplitude, the device will generally ‘clip’, creating very high distortion.
Almost any device will show very high distortion readings at maximum or
near-maximum amplitude.
The following graph shows common THD+N characteristics as a function of
amplitude:
Note that for some low-distortion devices, the flat area before clipping may shrink or
not exist at all. The THD+N measurement may be noise-dominated at all
amplitudes below the clipping threshold.
100% (0 dB)
THD+N
(Log Scale)
Amplitude
(Log Scale)
higher
lower
Fundamental has
reached noise floor
THD+N is
dominated
by noise
THD+N is
dominated
by distortion
Clipping
threshold
4
Oper
ati
o
n
Controlling the Analyzer : THD+N (Total Harmonic Distortion plus Noise)
Operation
Portable One Plus Access User's Manual
4-35
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