u4089b-m ATMEL Corporation, u4089b-m Datasheet - Page 7

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u4089b-m

Manufacturer Part Number
u4089b-m
Description
Multi-standard Feature Phone Circuit With Voice Switch
Manufacturer
ATMEL Corporation
Datasheet

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1. In speech condition, the system is supplied by the line
For line voltages below 1.9 V, the switches remain in their
quiescent state as shown in the diagram.
OFFSACOMP disables the group listening feature (SAI,
SA, SACL, AFS) below line currents of approximately
10 mA.
2. Selecting IMPSEL = high impedance activates all
Acoustic Feedback Suppression
Acoustical feedback from the loudspeaker to the handset
microphone may cause instability in the system. The
U4089B
Rev. A3, 06-Mar-01
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.
switches at the ES line.
microphone
speaker
Handset
Loud
Logarithmic
Figure 7. Basic voice-switch system
Attenuation
attenuation
attenuation
control
TX
RX
U4089B-M offers a very efficient feedback suppression
circuit which uses a modified voice switch topology.
Figure 7 shows the basic system configuration.
Two attenuators (TX ATT and RX ATT) reduce the criti-
cal 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
curve.
The system configuration for group listening, which is
realized in the U4089B-M, is illustrated in figure 8. TXA
and SAI represent the two attenuators; the logarithmic en-
velope detectors are shown in a simplified way
(operational amplifiers with two diodes).
Logarithmic
Hybrid
U4089B-M
12692
Line
7 (23)

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