U4089B-P_05 Datasheet, PDF (8/26 Pages) ATMEL Corporation – Monolithic Integrated Feature Phone Circuit

Figure 4-3 on page 7 provides a detailed diagram of the AFS (Acoustic Feedback Suppression).
Receive and transmit signals are first processed by logarithmic rectifiers in order to produce 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 4-4 illustrates the principal relationship between speaker amplifier gain (GSA) and atten-
uation 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 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.
Figure 4-4. 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)}
8 U4089B-P
4518C–CORD–10/05