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Application note # 7
Sample Champion

 

Average in time and frequency domains

TIME DOMAIN:

When performing MLS measures in noisy environments, a large part of the background noise (uncorrelated with MLS signal) is rejected automatically, but a small part still remains especially when the disturbance is louder than the MLS.
In these cases
an average of impulse responses may be useful to increase considerably the Signal/Noise Ratio.

Here is an example of impulse response measurement:

 

Figure 1 - Impulse response measured in a noisy environment

 

average of 16 measurements

Figure 2 - Same impulse response of figure 1 measured in a noisy environment (16 averages)

 

FREQUENCY DOMAIN:

Averaging in frequency domain may be useful not only to maximize S/N Ratio, but also to obtain a lower noise floor.

The following example plots frequency data from a measurement of 2 tones at 19 kHz and 20 kHz (typically used for evaluating intermodulation distortion) windowed in the time domain using a Full Blackman-Harris window. Figure 3 is obtained from a single measure, figure 4 from an average of 64 cycles. Note that the noise floor is much lower, when data are averaged!

 

Figure 3 - Frequency data from a single measure

 

Figure 4 - Frequency data from an average of 64 measures

 

 

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