If you see this graph, you should immediately suspect that the THD+N
reading is dominated by a significant component at the frequency of
the dropout.
The reason for this is that the notch (bandreject) filter tracks the
fundamental tone. When the fundamental tone is far away from the
interfering signal, the interfering signal is included in the THD+N
measurement. When the fundamental tone is at the same frequency as
the interfering signal, the notch filter removes the interfering signal as
well as the fundamental tone. Therefore, the interfering tone is not
included in the THD+N measurement at that frequency.
This can be a useful technique for locating a source of interference. If
you suspect interference at a certain frequency, make a THD+N
measurement with the fundamental at that frequency. If this
measurement is much lower than measurements made using other
fundamental frequencies, then there is interference at that frequency.
In some cases, you may see an erroneous dropout in a THD+N vs.
frequency sweep because of phase cancellation. This is caused by one
of the distortion products of the fundamental tone arriving out-of-phase
with an interfering tone and canceling it out. When the sweep is run
again, the amount of cancellation will vary significantly. If you see an
unexpected dropout in a THD+N sweep, it is a good idea to run it
several times. If it varies significantly (or disappears) with successive
runs of the test, then it is probably a cancellation problem. In this case,
the low THD+N reading is incorrect.
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.
Understanding THD + N FUNCTION Descriptions
4 Functions
4-18 ATS-1 Access User's Manual
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