PBL 388 12
Functional Description
Speech control section
Transmitter and Receiver
Channels
1
CTR
PBL 388 12
12
16
5
T
xout
R
xout
GND
Control
F3
F2
F6
F5
The transmitter and receiver channels
consist of three amplifying stages each,
F1, F2, F3 and F4, F5, F6. The inputs of
the amplifiers must be ac. coupled
because they are dc. vise at the internal
reference voltage (≈2V) level. F1 and F4
are fixed gain amplifiers of 30,5 dB and
15.5 dB respectively, while the rest of them
are of controlled gain type.The gain of F2,
F3 as well as F5 and F6 is controlled by
comparators. The comparator receives its
information partly from the summing point
of the transmitter, receiver and backgro-
und noise detectors at CMP input and
partly through the control input, CTR,
which controls the gain dynamics (25 or
50 dB). Amplifiers F2 and F3 have the
maximum gain when the transmitter
channel is fully open, consequently the
amplifiers F5 and F6 will have minimum
gain and vice versa. See figure 5 and figure
11.
4
13
11
6
15
V
+
+
2
3
-T
xin
-
14
R
F1
xin
F4
Ref.
+
T
xin
+
8
7
10
9
N
T
xDet
CMP
R
xDet
Det
R5
+
+
+
C4
C3
C1
C2
Figure 5. Passive networks setting the speech control function.
The positive input on transmitter and the
negative input on receiver channel has a
rather high input impedance. It renders a
good gain precision and noise
performance when used with low signal
source impedance. The differential input
of the transmitter channel can be used to
suppress unwanted signals in the
microphone supply, see figure 7. Also see
application 1.
PBL388 12
F
5
I
F
2
Ref.
120k
120k
100k
100k
F
4
F1
+
+
2
3
20k
Signal Detectors and
Comparator
3k
Rx
3k
Tx
14
16
~
~
V
V
Txin
Rxin
The signal detectors sense and rectify
the receiver and microphone signals to
opposite polarities referenced to the
internal reference voltage of approx. 2V.
The voltage at RxDet will go positive and
at TxDet negative in the presence of a
signal at the respective channel input. In
the idle (no signal) state, the voltages at
RxDet ,TxDet and CMP are equal to the
internal reference voltage. Signal at Txin
will result in an decreasing level at
Figure 6. Receiver and transmitter channel input arrangement.
TxDetout and hence also at CMP input.
6
ERICSSON [ ERICSSON ]
