u4090b-p ATMEL Corporation, u4090b-p Datasheet - Page 11

• Manufacturer Part Number: u4090b-p
• Description: U4090b-p Monolithic Integrated Feature Phone Circuit Emi Improved
• Manufacturer: ATMEL Corporation

4741C–CORD–11/05
Figure 3-6.
A detailed diagram of the AFS (acoustic feedback suppression) is given in
and transmit signals are first processed by logarithmic rectifiers in order to produce the enve-
lopes of the speech at TLDT and RLDT. After amplification, a decision is made by the differential
pair which direction should be transmitted.
The attenuation of the controlled amplifiers TXA and SAI is determined by the emitter current
IAT which consists of three parts:
I
Figure 3-7 on page 12
and attenuation of AFS (ATAFS). Both parameters can be adjusted independently, but the inter-
nal coupling between them has to be considered. The maximum usable value of GSA is 36 dB.
The shape of the characteristic is moved in the x-direction by adjusting resistor RATAFS, thus
changing ATAFS
reading the value which belongs to the actual gain GSA
reduced, the attenuation of AFS is automatically reduced by the same amount in order to
achieve a constant loop gain. Zero attenuation is set for speaker gains
GSA GSA0 = 36 dB – ATAFS
I
I
I
AT
ATAS
ATGSA
AGAFS
G = I
TX
RX
= I
ATAFS
AT
0.67 dB/µA
R
– I
R
LDR
LDT
ATGSA
sets maximum attenuation
decreases the attenuation when speaker amplifier gain is reduced
decreases the attenuation according to the loop gain reduction caused
by the AGA function
Acoustic Feedback Suppression by Alternative Control of Transmit and Speaker
Amplifier Gain
INLDR
INLDT
m
. The actual value of attenuation (ATAFS
– I
AGAFS
illustrates the principle relationship between speaker amplifier gain (GSA)
m
.
TLDT
TLDR
I
ATAFS
I
AT
R
TXA
ATAFS
I
ATAFS
ATGSA
a
a
), however, can be determined by
. If the speaker amplifier gain is
SAI
I
AGAFS
AGA
GSA
Figure
U4090B-P
I
GSA
3-6. Receive
AGA
11