8

U4089B-P

4518A–CORD–05/02

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

sets maximum attenuation

I

ATGSA

decreases the attenuation when speaker amplifier gain is reduced

I

AGAFS

decreases the attenuation according to the loop-gain reduction caused

by the AGA function

I

∆G = I

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

(ATAFS

gain GSA

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

Figure 9. Reducing Speaker Amplifier Gain Results in an Equal Reduction of

AFS Attenuation

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.

ATAFS (dB)

36 dB

ATAFS m

ATAFS a

GSAo

GSA a

GSA (dB)

not usable

RATAFS

RATAFS