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.
THD+N at 100% (or 0 dB)
If the THD+N reads 100% (or 0 dB), the amplitude of signal is the same before and
after the filter. Even if a device is creating a huge amount of distortion or interference,
the THD+N should be less than 100% because the signal before the filter should
include the fundamental, whereas the signal after the filter should not.
If the amplitude of the fundamental tone is below the noise floor, then both amplitude
measurements (before and after the notch filter) will be dominated by noise. The
amplitudes will be the same, and the THD+N will read around 100%.
Another common reason for a reading of 100% THD+N is that the notch filter is not
correctly tuned to the fundamental frequency. In this case, the signals before and after
the filter will be dominated by the fundamental tone, and the filter will be removing
nothing except a tiny, insignificant piece of the noise floor.
If your distortion reading is 100%, check the filter tuning mode. AUTO-TUNE should
tune correctly as long as there is sufficient signal amplitude for a valid frequency
reading. FIX-TUNE will give 100% readings unless the filter is tuned to the frequency of
the incoming fundamental tone. GEN-TRACK will give 100% readings if the frequency
of the incoming fundamental tone is different from the generator frequency.
4 Functions
Function Descriptions THD+N (Total Harmonic Distortion plus Noise)
ATS-1 Dual Domain User's Manual 4-19
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