u4090b-fn ATMEL Corporation, u4090b-fn Datasheet - Page 10

• Manufacturer Part Number: u4090b-fn
• Description: Monolithic Integrated Feature Phone Circuit
• Manufacturer: ATMEL Corporation

3. During ringing the supply for the system is fed into V
4. In an answering machine the chip is powered by an
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, wether 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
U4090B
10 (34)
via the ringing power converter (RPC). The only
functional amplifiers are found in the speaker
amplifier section (SAI, SA, SACL).
external supply via pin V
a posibility to activate all amplifiers (except the
transmit line
Loudspeaker
microphone
interface
Handset
B
. This application demands
TXA).
Preliminary Information
Selecting
Figure 5. Basic voice switch system
Log
IMP-
B
contr
TX
Att
Att
RX
Att
Acoustic feedback suppression
Acoustical feedback from the loudspeaker to the handset
microphone may cause instability in the system. The
U4090B offers a very efficient feedback suppression
circuit, which uses a modified voice switch topology.
figure 8 shows the basic system configuration.
the logarithmically compressed TX- and RX- envelope
curve.
The system configuration for group listening, which is
realized in the U 4090 B, is illustrated in figure 9. TXA
and SAI represent the two attenuators, whereas the
logarithmic envelope detectors are shown in a simplified
way (operational amplifiers with two diodes).
SEL = high impedance activates all switches at the ES
line.
Log
TELEFUNKEN Semiconductors
Hybrid
Rev. C1, 28-Oct-96
94 8956
Line