User`s manual

10.4 MEASURING FREQUENCY RESPONSE
In a step by step process we will deal with any single aspect that affects MLS & LOG
CHIRP measurement results. At first we deal with electrical measurements, leaving
acoustical as the last steps.
10.4.1 MEASUREMENT LEVEL
Opening the MLS & LOG CHIRP menu for the first time you will see a graph which
has frequency on its X-axis. Our first step will be measuring the response of an "A"
weighting filter. All the settings are left in their default state; we will take care when
measuring level only. Please ensure that the device being measured cannot be
damaged by the output level chosen for the actual measurement. In this example,
the device under test cannot be damaged with an output set to 0dB (-5.21dBV with
MLS signal, -2.21dBV with LOG CHIRP). Having ensured a safe level we connect
CLIO output A with the device input, CLIO Input A with the device output, then
enable input auto-range and click the Go button. As a result we get Fig.10.5
Audiomatica Srl MLS - Frequency Response 06/07/2001 10.15.54
CH A dBV Unsmoothed 51.2kHz 16K Rectangular File: fig1.mls
100 1k 10k 20k20 Hz
30.0
dBV
180.0
Deg
20.0 108.0
10.0 36.0
0.0 -36.0
-10.0 -108.0
-20.0 -180.0
CLIO
Audiomatica Srl MLS - Frequency Response 06/07/2001 10.51.17
CH A dBV Unsmoothed 51.2kHz 16K Rectangular File: fig2.mls
100 1k 10k 20k20 Hz
40.0
dBV
180.0
Deg
30.0 108.0
20.0 36.0
10.0 -36.0
0.0 -108.0
-10.0 -180.0
CLIO
Figure 10.5 and 10.6
The curve reaches 16dBV (6.3V) at 2.5kHz, which is quite a high level for our
device. Looking for trouble, we increase CLIO output to +6dB and measure again
obtaining Fig.10.6. The device went into saturation; in more emphatic terms it is
not linear any more. The whole MLS & LOG CHIRP process works on the assumption
the device is linear. If this is not the case, it is not easy for an inexperienced user to
understand what is going on just by inspecting the results. Checking the measuring
level is important, especially when the gain of the device under test is unknown.
You should use the Multi-Meter and the MLS & LOG CHIRP signal to accomplish this.
10.4.2 MLS & LOG CHIRP SIZE
The MLS & LOG CHIRP Size is the number of points that defines the MLS & LOG
CHIRP sequence. In terms of generating a digital signal these are the number of
samples before the signals is started again from the beginning. CLIO handles MLS &
LOG CHIRP sizes from 4k to 256k. These terms are inherited by the computer world
and are somewhat imprecise. The real size is the nearest power of 2, the 4k being
4096 points and the 256k 262144 points long (one less! for MLS, exactly this for
LOG CHIRP). The first important consequence of setting the MLS & LOG CHIRP size
is the length of the Impulse Response recovered which is always as long as the MLS
& LOG CHIRP itself. From the users point of view what matters is how long this
impulse is in terms of seconds. This in turns also depends on the Sampling
Frequency set. It is easily calculated as the size divided by the sampling frequency.
The default is 16k and 48000 Hz, that is 16384/48000=0.341 seconds. We will deal
with this in more detail later. For now it is enough to say that CLIO also uses this
120 10 MLS & LOG CHIRP