Changes with Frequency
Sometimes THD+N will be very consistent over a range offrequencies 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 ofsignificant hum.
For example, you may see a THD+N sweep that looks like this:
Ifyou 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 ofphase cancellation. This is caused by one ofthe 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 ofcancellation will vary
significantly. If you see an unexpected dropout in a THD+N sweep, it is a good
idea to run it several times. Ifit varies significantly (or disappears) with successive
runs ofthe test, then it is probably a cancellation problem. In this case, the low
THD+N reading is incorrect.
4
Oper
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Operation
Controlling the Analyzer : THD+N (Total Harmonic Distortion plus Noise)
4-34
Portable One Plus Access User's Manual
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