Figure 11. acoustic coupling measurement method, An168 – Cirrus Logic AN168 User Manual

Page 16

AN168

AN168REV2

The loop gain measurement procedure is as
follows:

1) Configure the CS6422 with its default configu-

ration, with the exception of the following:

a) Mic = ‘1’ or ‘0’, depending on whether the

internal mic preamp is used or not

b) TSD = RSD = HDD = ‘1’, transmit and re-

ceive suppressors and half-duplex mode are
disabled

c) ACC = NCC = 'cleared', echo cancelers are

forced to a cleared state to prevent updates

d) AECD = NECD = ‘1’, echo cancelers are

disabled

e) TVol = +12 dB

f) NSdt = -12 dB

g) RVol = 0 dB

2) Adjust RVol until the system is just on the

verge of howling. At this point the loop gain is
0 dB, and the loss between AO and APO (out-
side the CS6422) is equal to the RVol value.

The register settings to accomplish the above are as
follows:

reg 0: 4xa0 (or cxa0 if internal mic preamp is
used); “x” is the value of RVol

reg 1: 26a2

reg 2: 0004 (default)

reg 3: 0006 (default)

reg 4: 0008 (default)

reg 5: 033a

RVol contains a 4-bit value that results in a gain
given by the following equation:

For example, if the RVol value required to make
the system howl is ‘5’, then the loss between AO
and APO is (30 - 5 * 3 = 15 dB)

Physical law ensures that the system will howl at
the frequency of maximum coupling. Furthermore,

Speaker

Driver

Mic

Preamp

CS6422

APO

TVol - Transmit

Volume

NSdt - Network

Sidetone

Air Coupling

Adjust RVol until howling begins; AC = 0 - RVo l - Mic -Spkr

0dB

-?dB

30dB

12dB

-12dB

RVol - Receive

Volume

+?dB

Adjust

isconn

ect

two

Figure 11. Acoustic Coupling Measurement Method

Gain dB

(

)

30dB

RVol

3dB

(

)

–