6

U4089B-M

4570A–CORD–04/03

Figure 5. Functional Blocks for Power Supply

1.

In speech condition, the system is supplied by the line current. If the LIDET-block

detects a line voltage above the fixed threshold (1.9 V), the internal signal VLON

is activated, thus switching on all blocks of the chip.

For line voltages below 1.9 V, the switches remain in their quiescent state as shown

in Figure 5.

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 at the ES line.

Acoustic Feedback

Suppression

Acoustical feedback from the loudspeaker to the handset microphone may cause insta-

bility in the system. The U4089B-M offers a very efficient feedback suppression circuit

which uses a modified voice switch topology. Figure 6 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 com-

pressed TX and RX envelope curves.

The system configuration for group listening, which is realized in the U4089B-M, is illus-

trated in Figure 7. TXA and SAI represent the two attenuators; the logarithmic envelope

detectors are shown in a simplified way (operational amplifiers with two diodes).

7V

MIC, DTMF

AGA, RA

TX MUTE

MUT REC, STBAL

RECATT

AFS

SAL, SA

SACL

OFFSA

COMP

TXA

TXACL

LIDET

Voltage

regulator

Power

supply

IMPED

CONTR

IMPSEL

ES