It is most common to limit the bandwidth to 80 kHz, since this will reject
high-frequency noise while passing the second and third harmonics of signals up to
22 kHz, the top ofthe audible frequency range.
You may want to limit the lower band limit to remove hum from the measurement.
Using the 400 Hz highpass filter will eliminate the hum products while passing all
fundamentals, interference, distortion products, and noise above 400 Hz.
Following is an example ofthe use ofvarious bandwidth-limiting filters. The
spectrum ofthe original signal may look like this:
After the bandpass filter eliminates the 1 kHz fundamental tone, we are left with the
following:
This signal contains contributions from harmonic distortion (at 2 kHz, 3 kHz, and 4
kHz), an interfering tone (at 490 Hz), random noise, and 60 Hz hum. The THD+N
measurement includes all ofthese, and reads 0.229 %.
Ifwe use the 400 Hz bandpass filter to eliminate the hum, the spectrum ofthe signal
then looks like this:
The THD+N now measures 0.185 %. From this, we can determine that the hum
product was contributing 0.044 % to the THD+N reading.
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Operation
Controlling the Analyzer : THD+N (Total Harmonic Distortion plus Noise)
4-32
Portable One Plus Access User's Manual
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