User`s manual

10.7 MLS Vs. LOG CHIRP
As anticipated in the introduction, some advise are given to help choosing between
MLS and LOG CHIRP stimuli. Both approach are valid and bring to equivalent
results. In both cases the device we want to measure is assumed to be Linear and
time Invariant. This assumption while reasonably true in general cases in never met
in absolute terms. There is always a certain degree of non linearity and, in less
degree, a time variance. The two approach shows different sensitivity to these
facts. Advantage of MLS are that is that much less computing requiring, historically
much more widespread and known and has a flat frequency energy distribution.
The latter feature can be an advantage or not depending on situation/DUT.
Figure 10.39
In the above figures the spectrum of both signals is showed. As usually signal to
noise ratio get worse at lower frequency, the higher signal energy that LOG CHIRP
has here is an advantage for S/N ratio. If the DUT does not appreciate low
frequency high energy content, i.e. a tweeter, the advantage become a
disadvantage.
Also, there is an important difference on how non linearity affects the two
procedure. In MLS distortion transforms itself in noise spread over the whole
134 10 MLS & LOG CHIRP