PBL 388 14
Functional Description
Speech control section
Transmitter and Receiver
Channels
+
GND
A
The transmitter and receiver channels
consistofthreeamplifyingstageseach,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 29.5 dB and 15.5 dB
respectively, while the rest of them are of
controlled gain type amplifiers.The gain of
F2, F3 as well as F5 and F6 is controlled by
the comparator. Ac. loading the channel
outputs F3 and F6 will lessen the dc.
current consumption, maximum load 10
kΩ. The output capacity can be increased
somewhat in case needed, by coupling a
10 kΩ resistor from the respective output
pin directly to ground (before the optional
capacitor).The comparator receives its in-
formation from the summing point of the
transmitter,receiverandbackgroundnoise
detectors at CMP input. The control input
CTR, controlsthegain dynamics(25or50
dB). Amplifiers F2 and F3 have the maxi-
mum 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.
LSP
V
A
VOL
LL
22
21
19
18
20
23
LSP
in
GND
PBL 388 14
17
11
24
CTR
4
T
xout
R
xout
16
V
+
+
Control
F3
F
6
3
5
13
11
F2
F5
15
14
1
2
-T
xin
-
-R
xin
xin
F
1
F
4
Ref.
7
+
T
xin
+
+
R
+
8
6
10
9
N
T
xDet
R
Det
CMP
xDet
R5
+
C1
+ C2
+ C3
C4
The positive input on each channel
has a high input impedance. It renders a
goodgainprecisionandnoiseperformance
when used with low impedance signal
source . The negative input of the receiver
channel should be returned to ground with
a capacitor. The differential input of the
transmitter channel can be used to sup-
press unwanted signals in the microphone
supply, see figure 7. Also see application.
Figure 5. Passive networks setting the speech control function.
PBL388 14
Signal Detectors and the
Comparator
F5
I
F2
Ref.
Thesignal detectorssenseandrectify
the receiver and microphone signals to
oppositepolaritiesreferencedtotheinternal
referencevoltageofapprox.2V.Thevoltage
120k
20k
120k
100k
100k
F4
F1
+
+
at Rx
will go positive and at Tx
3k
20k
15
14
Det
Det
negative in the presence of a signal at the
respective channel input. In the idle (no
1
Rx
3k
Tx
2
17
signal) state, the voltages at Rx
,Tx
Det
Det
~
~
V
V
Txin
Rxin
and CMPareequaltotheinternal reference
voltage. Signal at Tx will result in a
in
decreasing level at Tx
also at CMP input.
and hence
Detout
Figure 6. Receive and transmit channel input arrangement.
6
ERICSSON [ ERICSSON ]
