User guide

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CHAPTER 7
Using SIM 3
7.11. MEASURING THD
SIM 3 measures THD by looking at the level at the cursor, then searching for its first 9
harmonics (including noise). The levels of those harmonics are summed, and that level is
calculated as a percentage of the fundamental’s level. A THD measurement is only valid if the
signal being measured is a sine wave, and if the cursor is on the fundamental frequency of
that sine wave.
7.11.1. Standard THD
The standard THD measurement in SIM 3 goes to one decimal point (0.1%), and can be used
with a sine wave on any desired frequency. Its value is shown in the THD field above the
graph. To make this frequency selectability possible, the measurement uses a broader “flat-
top” window, with very attenuated side lobes that are still significantly above the analyzer’s
extremely low noise floor (the side lobes in the high-precision THD measurements are at
approximately the level of the noise floor).
The trade-off between the two THD measurements available in SIM 3 is that the standard
THD gives the ability to measure THD at any arbitrary frequency, but at the cost of increased
side lobes (and therefore lower precision).
NOTE: Speaker distortion measurements would be done with the standard
lower precision THD setting, which can use a signal from an external source
through the Console.
7.11.2. High-Precision THD
The high-precision THD measurement in Line Spectrum provides a highly accurate distortion
value to three decimal points (0.001%), which is displayed in a field at the top of the graph.
This measurement is selected via the Spectrum and Meter Settings dialog by right-clicking
on the spectrum measurement screens or selecting Settings > Spectrum Settings.
When doing high-precision THD measurements, the SIM 3 sine generator must be used, and
is limited to a certain set of frequencies up to 8 kHz. The high-precision THD measurement
uses a different windowing function with lower side lobes than the standard-precision one,
providing the enhanced THD accuracy but without the ability to select any desired frequency
on which to center the THD measurement.