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The fundamental tone has been reduced to a level where it will be

insignificant. The filtered signal is then measured with another

voltmeter, and the reading is presented as a ratio of the two voltmeter

readings.

Notice that the final measurement is wideband. It measures the total of

all the noise sources within the frequency range of interest. It is not

specific to the frequency of the fundamental tone. While some devices

may create more or less harmonic distortion for different fundamental

frequencies, the other sources of unwanted sound will contribute to the

reading no matter what fundamental frequency is used.

Surprises and Pitfalls in THD+N Measurements

What Signal is Dominant?

While all types of unwanted sounds are included in the THD+N

measurement, they may not all contribute to the overall reading. Like

all voltmeter readings on signals that include several frequency

components, the highest-amplitude components will dominate the

reading.

When comparing two individual signal components such as

interference tones or distortion products, the limit is commonly

considered to be 10 dB. If one component is 10 dB above the other,

only the higher component will be significant to an amplitude reading.

However, when there are many lower components, the combination

may become significant. For example, random noise can be a

significant contribution to a THD+N measurement even though its

amplitude at any particular frequency is well below other products.

Since noise exists at every frequency, and each frequency has a tiny

effect on the overall reading, the combination of all the noise may be

quite significant.

Often THD+N tests will be used in conjunction with noise-only tests to

be certain that a problem is not hiding underneath a more significant

contributor.

The section Isolating Sources of THD+N (page 4-37) explains some

strategies for determining which unwanted signal is dominating the

reading.

FUNCTION Descriptions Understanding THD + N

4 Functions

ATS-1 Access User's Manual 4-13

Audio Precision ATS-1 Access User Manual

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