U4089B-P Datasheet, PDF (8/26 Pages) ATMEL Corporation – Monolithic Integrated Feature Phone Circuit

A detailed diagram of the AFS (Acoustic Feedback Suppression) is given in Figure 8.
Receive and transmit signals are first processed by logarithmic rectifiers in order to pro-
duce the speech envelopes 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 is comprised of three parts:
I_ATAFS
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 9 illustrates the principal relationship between speaker amplifier gain (GSA) and
attenuation of AFS (ATAFS). Both parameters can be adjusted independently, but the
internal coupling between them has to be considered. The maximum GSA value to be
used 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 the 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 automati-
cally 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.
Figure 9. Reducing Speaker Amplifier Gain Results in an Equal Reduction of
AFS Attenuation
ATAFS (dB)
ATAFS _m
ATAFS _a
RATAFS
RATAFS
not usable
GSA _o
GSA _a
36 dB _{GSA (dB)}
Operating Range of
Speaker Amplifier
The basic behavior is illustrated in Figure 10. Actual values of ILON/ILOFF vary slightly
with the adjustment of the DC characteristics and the selection of the internal line
impedance.
8 U4089B-P
4518A–CORD–05/02