PBL 3881
Functional Description
Speech control section.
Transmitter and Receiver
Channels
-C
LS
+L
The transmitter and receiver 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 because they are dc. vise at the
internal reference voltage (≈1.9 V) level.
F1 and F4 are fixed gain amplifiers of 30
dB and 20 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
21 17
19
20
22
16
R
E
–
+
B
in
GND
18
R
DC
5
T
xout
PBL 3881
14
CTR
10
R
xout
Control
comparators receive their information
partly from the summing point of the
transmitter, receiver and background
noise detectors at CMP input and partly
through the control input, CTR, which
controls the gain dynamics (0-50 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 8. and figure 15.
+
–
–
+
F
3
F
6
15
4
V
+
F2
F
5
F
2 in
11
13
12
3
1
F
4out
F
1out
V
Ref
-T
xin
-
-R
1.9V
xin
xin
F
1
F4
2
+
T
xin
+
+
+
R
7
6
8
xDet CMP
9
The positive input on each channel has
a high input impedance. It renders a
good gain precision and noise
N
T
R
xDet
Det
R1
R5
R2
performance when used with low signal
source impedance. The negative input of
the receiver channel should be returned
to ground with a capacitor. The differen-
tial input of the transmitter channel can
be used to suppress unwanted signals in
the microphone supply, see figure 10.
Also see application 1.
R6
R3
R4
C1
+
+
C4
C3
C2
The output of the transmitter channel
input amplifier (F1) and the input of (F2)
are made accessible in order to allow
filtering in the channel. The output of F4
can be used to implement an optional
AGC (Automatic Gain Control) system in
the receiver channel. The maximum dyn-
amic range of the AGC can be made the
same as the gain of F4. See figure 11.
Figure 8. Passive networks setting the speech control function.
I
F
5
F
2
1
4
3
11
AGC
Signal Detectors and
Comparators
Ref.
100k
100k
100k
100k
+
F
4
F
The signal detectors sense and rectify
the receiver and microphone signals to
opposite polarities referenced to the
internal reference voltage of approx.
1.9 V. The voltage at RxDet will go
negative and at TxDet positive in the
presence of a signal at the respective
channel input. In the idle (no signal)
state, the voltage at RxDet ,TxDet and CMP
is equal to the internal reference voltage
+
1
2
13
12
Rx
Tx
V
3.2k
10k
16
V
Txin
Rxin
Figure 9. Receive and transmit channel input arrangement.
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