Stanford Research Systems DS360 Operating Manual And Programming Reference page 100

6-6 Performance Tests
Total harmonic distortion can be calculated from the following formula:
T.H.D. (dB) = 20 log (
where 2nd, 3rd, 4th, ... nth are the values of the corresponding harmonics in dB relative to the
fundamental.
DS360 Ultra Low Distortion Function Generator
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√
-1 2 -1
(log (2nd)) + (log (3rd))
2 -1 2
+ (log (4th)) + ... + (log
-1 2
(nth)) )