User`s manual

9.5 FFT AND RTA OPERATION
The FFT and RTA measurements (and also Multi-meter ones, see Chapter 8) differ
from MLS and Sinusoidal ones in the fact that they are interactive; the user has
control over measurement time and generated stimuli. You may also obtain
answers about unknown signals from them, without any need for generating a
stimulus; or you may leave this job to others, similar to when you measure an
audio chain relying on the test signals contained in a CD-ROM. One effect of this is
that, strictly speaking, FFT measurements may lead to less precise results if
compared to other techniques; the possibility of injecting a synchronous MLS
sequence at the beginning of the same audio chain mentioned before is surely a
better approach even if, in the vast majority of cases, unfeasible.
FFT and RTA power depends not only on the measurements settings themselves but
also on the generated signals. Please refer to chapter 7 for a detailed description
of the signal generator and its many capabilities.
When stimulating any external device with CLIO (see 4.8.2 and 4.8.3 for basic
connections) you may choose a limited bandwidth signal (like a single sinusoid) or a
wide bandwidth signal as a noise; in the first case you have the possibility of
analyzing the harmonic content of the output spectrum while in the second case
you may evaluate the frequency response of the device under test. A different
stimulus, about halfway between the two cases just mentioned, is a logarithmic
chirp swept across some octaves (like a chirp covering four octaves from 50 to
800Hz); in this case you are able to analyze both the response plus unwanted
effects like distortion and noise produced by the D.U.T..
When using the FFT narrowband analyzer it is possible to achieve a flat response of
the analyzing chain using white noise or similar signals whose energy content varies
linearly with frequency; among these MLS, All-tones signals or linear Chirps.
When using the RTA octave bands analyzer it is possible to achieve a flat response
of the analyzing chain using a signal whose energy content varies logarithmically
with frequency; among these we find pink noises or logarithmic Chirps.
Besides the choice of the stimulus it is very important to achieve proper
synchronization between the generated signal and the acquisition; this will lead
to optimum performances avoiding the use of data windows and minimizing any
spectral leakage that may occur. Synchronization can be achieved defining the
stimulus in a particular manner or by means of proper triggering (see later internal
trigger).
If you are generating a sinusoid choose a frequency that is an integer multiple of
the frequency bin (i.e. sampling frequency divided FFT size) or let CLIO calculate it
setting “FFT bin round” in the generator input form (see chapter 7). As an example
we would like to play and analyze a 1kHz sinusoid using a 64k FFT @ 48000Hz
sampling; the frequency bin associated is 0.73Hz and the nearest spectral line to
1kHz is the 1365th one at 999.75Hz. If you simply generate a 1kHz sinusoid
without rounding it to the nearest bin you obtain the analysis of fig. where it is
evident that CLIO is capable of outputting a highly precise 1000.0Hz sinusoid but
it is also evident the spectral leakage caused by this choice.
9 FFT, RTA AND “LIVE” TRANSFER FUNCTION 103