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 Operation
Operation Controlling the Analyzer : THD+N (Total Harmonic Distortion plus Noise)
4-36 Portable One Plus Access User's Manual
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