U4090B-P Datasheet, PDF (11/35 Pages) ATMEL Corporation – Monolithic Integrated Feature Phone Circuit EMI Improved

U4090B-P
Figure 3-6. Acoustic Feedback Suppression by Alternative Control of Transmit and Speaker
Amplifier Gain
TX
R_LDT
INLDT
RX
RLDR
INLDR
TLDT
TXA
SAI
IAT
I_ATAFS
AGA
I_AGAFS
AGA
IGSA
TLDR
R_ATAFS
I_ATGSA
ATAFS
GSA
A detailed diagram of the AFS (acoustic feedback suppression) is given in Figure 3-6. Receive
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_ATAS
I_ATGSA
I_AGAFS
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
I_AT = I_ATAFS – I_ATGSA – I_AGAFS
∆G = I_AT × 0.67 dB/µA
Figure 3-7 on page 12 illustrates the principle relationship between speaker amplifier gain (GSA)
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_m. The actual value of attenuation (ATAFS_a), however, can be determined by
reading the value which belongs to the actual gain GSA_a. If the speaker amplifier gain is
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_m.
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