U4089B Datasheet, PDF (6/25 Pages) ATMEL Corporation – MONOLITHIC INTEGRATED FEATUREPHONE CIRCUIT

Figure 5. Functional Blocks for Power Supply
V_L
Power
supply
Voltage
regulator
V_B
7V
V _MP
LIDET
V _LON
ES _IMPED
CONTR
IMPSEL
TXA
TXACL
OFFSA
COMP
SAL, SA
SACL
AFS
MIC, DTMF
AGA, RA
TX MUTE
MUT REC, STBAL
RECATT
Acoustic Feedback
Suppression
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.
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).
6 U4089B-M
4570A–CORD–04/03