Cirrus Logic AN168 - Manual (Page 18)

AN168
18 AN168REV2
There are two tests which should be performed, a
Frequency Sweep distortion test and a Buzz test.
1.5.2.1 Frequency Sweep Test
In this test, a piece of automated test equipment
sweeps a sinewave between 100 Hz and 4 kHz,
similar to the Frequency Response Method of
testing acoustic coupling. In this test, the input to
the analyzer is analyzed for relative THD+N by
filtering with a high-Q notch filter at the
fundamental frequency of the input. Additionally, a
C-message filter or a high-order (5 poles or more)
4 kHz low-pass filter should be applied to keep
out-of-band noise from corrupting the
measurement.
The resulting curve shows relative THD+N vs.
Frequency. An example is shown in Figure 13.
This curve, although useful, is not complete
because it lacks information about the coupling
strength at each frequency. A relative THD+N
reading can be degraded by an increase in the
distortion component or a decrease in SNR (which
can be caused by a decrease in signal level or an
increase in noise level). The residual echo level is
dependent only on the amount of distortion. An
SNR degradation due to a decrease in signal level
means that there is no echo at those frequencies to
cancel.
The graph that we need is a relative THD+N curve
that is weighted with the acoustic coupling
information. This curve can be constructed by
normalizing the acoustic coupling curve by shifting
it vertically such that the maximum value is set at
0 dB then adding this normalized curve to the
relative THD+N curve. An example is shown in
Figure 14. Keep in mind that the goal is to have this
weighted distortion curve below 2% THD, or
-34 dB at all points.
These curves were constructed by saving the data
in text format to a file, then importing the
information into a spreadsheet for normalization
and addition.
1.5.2.2 Buzz Test
The distortion measurement method above is
designed to detect harmonic distortion resulting
from clipping at the speaker driver, the speaker, or
the mic preamp. This method is not good at
measuring non-harmonic distortion that results
when the speaker induces mechanical vibrations in
its housing. We refer to these induced vibrations as
‘buzzing’.
Relative THD+N (dB)
-60
-50
-40
-30
-20
-10
0
0 500 1000 1500 2000 2500 3000 3500 4000
Frequency (Hz)
Figure 13. Relative THD+N