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U4089B-PFN Datasheet(PDF) 8 Page - ATMEL Corporation |
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U4089B-PFN Datasheet(HTML) 8 Page - ATMEL Corporation |
8 / 26 page 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 AT = IATAFS - IATGSA - IAGAFS ∆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. T he actual val u e of the attenuatio n (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 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 |
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