U4089B-P_05 Datasheet, PDF (6/26 Pages) ATMEL Corporation – Monolithic Integrated Feature Phone Circuit

1. In speech condition, the system is supplied by the line current. If the LIDET block
detects a line voltage above the fixed threshold (1.9V), the internal signal VLON is acti-
vated, thus switching on all blocks of the chip.
For line voltages below 1.9V, the switches remain in their quiescent state as shown in Figure
3-2.
OFFSACOMP disables the group listening feature (SAI, SA, SACL, AFS) below line currents of
approximately 10 mA.
2. Selecting IMPSEL = high impedance activates all switches on the ES line.
4. Acoustic Feedback Suppression
Acoustical feedback from the loudspeaker to the handset microphone may cause instability in
the system. The U4089B-P offers a very efficient feedback suppression circuit which uses a
modified voice switch topology. Figure 4-1 shows the basic system configuration.
Two attenuators (TX ATT and RX ATT) reduce the critical loop gain by introducing an externally
adjustable amount of loss either in the transmit or in the receive path. The sliding control in block
ATT CONTR determines whether the TX or the RX signal has to be attenuated. The overall loop
gain remains constant under all operating conditions.
Selection of the active channel is made by comparison of the logarithmically compressed TX
and RX envelope curves.
The system configuration for group listening, which is realized in the U4089B-P, is illustrated in
Figure 4-2. TXA and SAI represent the two attenuators; the logarithmic envelope detectors are
shown in a simplified way (operational amplifiers with two diodes).
Figure 4-1. Basic Voice-switch System
Handset
microphone
TX
attenuation
Logarithmic
Attenuation
control
Hybrid
Line
Loud
speaker
Logarithmic
RX
attenuation
6 U4089B-P
4518C–CORD–10/05