Following is the spectrum of a different THD+N measurement. This one comes from a
device that generates a significant amount of high-frequency noise. The THD+N
measures 0.057 % with a lower band limit of 400 Hz and a high band limit of
300 kHz. The spectrum is shown after the bandreject filtering, which has removed the
1 kHz fundamental.
This measurement is dominated by the high-frequency noise, although the distortion
products (at 2 kHz, 3 kHz and 4 kHz) are probably contributing. Next, we can invoke
the 22 kHz lowpass filter. The spectrum then looks like this:
The high-frequency noise has been eliminated, and the measurement is now
dominated by the distortion products. The THD+N now measures 0.0390 %, which is
precisely the theoretical THD for these distortion products. The contribution of the
remaining noise (between 400 Hz and 22 kHz) is negligible.
20 50 100 200 500 1k 2k 5k 10k 20k 50k
Hz
-140
+20
-120
-100
-80
-60
-40
-20
+0
d
B
r
A
100k 200k 300k
Figure 4-15. Device with HF noise, 1 kHz fundamental filtered
-140
+20
-120
-100
-80
-60
-40
-20
+0
d
B
r
A
20 50 100 200 500 1k 2k 5k 10k 20k 50k
Hz
100k 200k 300k
Figure 4-16. Same device with 22 kHz lowpass filter added
4 Functions
THD+N (Total Harmonic Distortion plus Noise) Function Descriptions
4-16 ATS-1 Dual Domain User's Manual
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