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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.

Isolating Sources of THD+N

Because the THD+N reading may contain several different types of

unwanted sounds, it may sometimes be helpful to be able to isolate

which types of sounds are contributing to the measurement.

First, check your device to make certain that the amplitudes are correct

and that there are no other sound sources making contributions to the

output. Many THD+N problems are the result of this sort of error.

Then limit the low-frequencies using the 400 Hz highpass filter. This

filter will eliminate the hum, which is usually at 50 Hz or 60 Hz and

multiples of this frequency. If you see a significant difference in the

THD+N reading when you engage the 400 Hz filter, then a significant

amount of hum is contributing to the reading.

Next, change the reading units to volts (or any other unit besides % or

dB). This will reconfigure the instrument to read only the filtered

amplitude. Check the THD+N reading and then turn off the generator.

If the reading changes significantly, then a significant amount of

distortion is contributing to the reading.

Understanding THD + N FUNCTION Descriptions

4 Functions

4-20 ATS-1 Access User's Manual

Audio Precision ATS-1 Access User Manual

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