Cirrus Logic AN168 - Manual (Page 17)

AN168
AN168REV2 17
the frequency of the howl itself is the frequency at
which the loop gain, whose frequency response is
dominated by the acoustic coupling, is maximum.
1.5.1.2 Frequency Response Method
Measuring the frequency response of the acoustic
path in the target environment is strongly
recommended. In general, the flatter the frequency
response, the better the performance of the echo
canceler.
In this method, an automated piece of test
equipment that contains a variable-frequency sine
wave generator coupled with an RMS
measurement device, such as an Audio Precision
System One or System Two, or a Rohde and
Schwarz UPL or UPD is used. Automated
equipment is recommended over a manual sine
wave generator due to the number of frequency
points required (100 or more) and the difficulty of
measuring the mic signal due to constructive and
destructive interference.
The measurement device’s generator can be
connected to the input of the speaker driver or to
the NI pin of the CS6422 through a DC-blocking
capacitor. If connected to the NI pin, RGain and
RVol should each be set to 0 dB, RSD and HDD
should be set to ‘1’, and NCC should be set to
‘cleared’. If connected to the speaker driver input,
the connection between the AO pin and the speaker
driver should be broken.
The sine wave generator should be configured to
output a 1 Vrms (0 dBV) sine wave that sweeps
between 100 Hz and 4 kHz, log distribution is
preferred over linear, and samples at least 100
points, preferably 200 or more for this range.
The return signal can be monitored either at the
APO pin or the NO pin of the CS6422. If monitored
at the NO pin, TSD should be set to ‘1’, and ACC
should be set to ‘cleared’. In either case, the Mic bit
should be set to ‘1’ if the internal mic preamp is
used, or ‘0’ if the internal mic preamp is not being
used.
An example acoustic coupling plot is shown in
Figure 12.
1.5.2 Acoustic Distortion
Earlier in this note, we described a process by
which we could limit the distortion in the acoustic
path by designing the electrical system carefully. It
is still necessary to test the distortion to verify that
the distortion level is below 2% THD.
Acoustic Coupling (dB)
-80
-70
-60
-50
-40
-30
-20
-10
0
0 500 1000 1500 2000 2500 3000 3500 4000
Frequency (Hz)
Figure 12. Example Acoustic Coupling Frequency Response