PBL 3786
Functional Description
Speech Control Section
Transmit and receive channels
The transmit and receive channels
consist of three amplifying stages each,
F1, F2, F3 and F4, F5, F6. The inputs
and outputs of the amplifiers must be
AC-coupled.
F1 and F4 are fixed-gain amplifiers of
30 dB and 20 dB respectively, while the
others are of controlled-gain type.
The gain of F2 and F3, as well as F5
and F6, is controlled by comparators.
The comparators receive their informa-
tion partly from the summing point of the
transmit, receive, and noise detectors;
the CMP input; and partly through the
control input CTR, which controls the
dynamics range. Amplifiers F2 and F3
have the maximum gain when the
transmit channel is fully open, conse-
quently F5 and F6 will have minimum
gain. When F5 and F6 have maximum
gain, the receive channel is fully open
and the amplifiers F2 and F3 are at
minimum gain. See figure 10 and 11.
Signal detectors and comparators
The signal detectors sense and rectify
the signals to opposite polarities,
referenced to the internal reference
voltage of approximately 1.9 V. The
voltage at Rx
Det
will go negative and at
Tx
Det
positive, in the presence of a signal
at the respective channel input, Tx
in
and
Rx
in
.
In the idle (no signal) position, the
voltage at Rx
Det
, Tx
Det
, and consequently
CMP is equal to the internal reference
voltage. The inputs at pins 2, 6 and 9 as
well as the outputs at pinns 3, 4, 5, 7
and 8 are biased DC vise to the
reference voltage.
The voltages Rx
Det
and Tx
Det
are used
to control the gain setting in the
respective channel through comparators,
where the CMP input is the summing
point.
A signal at Tx
In
will result in an
increased level at Tx
Det
and hence also
at CMP. The comparators will now
increase the gain in the transmit channel
and decrease it in the receive channel.
Signal at Rx
In
will affect the levels and
gain setting in the opposite way.
Maximum input current to CMP is 1
µA.
The attack and decay times of the
transmit and receive channels are set
+L
20
LS2
LS1
R
Vin
PWMµPC
14
HFµPS
13
LFµPA
12
18
15
17
B
in
-C
R
DC
R
E
CTR
Tx
out
1
22
19
21
10
3
+
–
Control
Control
16
GND
PBL 3786
+
+
–
–
F6
8
Rx
out
F3
V
+
11
F2
F5
Tx
in
2
F1
5
N
det
R1
R6
R5
+
C4
+
C3
+
C1
C2
R3
R4
4
Tx
det
6
CMP
7
V
ref
Ref.
≈1.9
V
≈
1,9V
F4
9
F4
Rx
in
Rx
det
R2
Figure 10. Block diagram.
dB
Transmit
gain = ---------
V = V
+
CTR
Receive
gain = ____
dB
30
40
20
30
10
20
0
10
-10
0
V =V
+
- 0.6 V
CTR
-60
-40
-20
0
20
40
60
-20
V
CMP
-V
REF
mV
Figure 11. Transmit- and receive- channel gain.
7
ERICSSON [ ERICSSON ]
