Surprises and Pitfalls in THD+N Measurements
What Signal is Dominant?
While all types ofunwanted 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. Ifone 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 ofall 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 ofTHD+N’ (page 4-37) explains some strategies for
determining which unwanted signal is dominating the reading.
Selecting Bandwidth
Selection of a bandwidth-limiting filter is often of critical importance in THD+N
measurements. This is mainly for two reasons:
.
Noise exists at every frequency. The wider the bandwidth of the
measurement, the more noise is included.
.
Limiting the bandwidth can eliminate other components, which you may or
may not want included in the measurement.
The amount ofnoise included in the measurement will be directly related to the
bandwidth. Ifyou are more concerned with distortion and interference products
than noise, it is wise to set the upper band limit to allow the highest component of
interest but reject noise above that frequency. Most mechanisms that create
harmonic distortion concentrate most ofthe distortion in the second and third
multiples (harmonics) of the fundamental. Therefore, for most devices it is
reasonable to limit the bandwidth to around four times the highest fundamental
frequency of interest.
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Oper
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Controlling the Analyzer : THD+N (Total Harmonic Distortion plus Noise)
Operation
Portable One Plus Access User's Manual
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