Changes with Frequency
Sometimes THD+N will be very consistent over a range of frequencies except that it
will ‘drop out’ at one frequency, giving a very low reading. This is an indication that the
THD+N reading is dominated by significant hum or interference at that frequency. This
is common when using a fundamental at 50 or 60 Hz in the presence of significant hum.
For example, you may see a THD+N sweep that looks like this:
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.
Figure 4-17. THD + N with interfering signal at same frequency as fundamental
4 Functions
Function Descriptions THD+N (Total Harmonic Distortion plus Noise)
ATS-1 Dual Domain User's Manual 4-17
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