PBL 388 12
the transmitter channel and decrease it in
the receiver channel. Signal at Rxin will
do vice versa. The voltages RxDetout and
TxDetout control thus the gain setting in
respective channel through the
comparators using the CMP input as a
summing point with an input current of
less than 1µA. The attack and decay times
for the signals RxDetout and TxDetout are
controlled by individual external RC-
networks. The attack time in the receiver
channel is set by C2 together with C1 and
either by the maximum current capability
of the detector output or it with R2 added.
The transmitter channel works likewise.
See fig. 7.
The decay time in the receiver and trans-
mitter channels is set by C2 and C3
respectively. The resistor in the time cons-
tant is formed by an internal 200kΩ resistor
in parallel with the external resistors R3
and R4 respectively. The influence of
eventual R1 and R2 can be omitted.
The text above describes the case when
only one channel is open at a time and
there is a distinctive pause between sign-
als at receiver and transmitter channel
inputs so the circuit will have time to reach
its idle state. See fig.14 A) to E). If one of
the channels gets an input signal
immediately after the signal has
disappeared from the other channel input
the effective decay time, as the CMP in-
put sees it, will be shorter than in the first
case. See fig.14 F) to G). The capacitor
C4 at CMP - input sets the speed of the
gain change in the transmitter and recei-
ver channels. The capacitors C2 and C3
should be dimensioned for a charging time
of 0.5 - 10mS and for a discharge time of
150 - 300 mS. The question of switching
times being a highly subjective proposi-
tion, is in large dependent of the language
being spoken in the system, caused by
the varying sound pressure picture of the
different languagues. A hysteresis effect
is achieved in the switching since the level
detectors sense the signals after F2 and
F5 respectively (F2 and F5 are affected
by the gain setting). For example: If the
transmitter channel is open (maximum
gain), a smaller signal at Txin is enough
to keep the channel open than would
benecessary to open it when the receiver
channel is open. The output swing of the
level detectors is matched for variations in
the supply voltage. The detectors have
dB
Transmit
gain = ____
Receive
gain = ---------
dB
30
40
20
30
VCTR=V+
VCTR=V+
10
0
20
10
-10
0
VCTR=open
VCTR=open
-20
V
CMP
-V
REF
mV
V
CTR
= V
REF
-60
-40
-20
0
20
40
60
Figure 11. Transmit and receive gain as a function of V
CMP
and V
CTR
.
Rxdet
Txdet
A
B
E
Full recieve level
C
CMP
Full transmit level
D
F
G
Figure 12. Timing of the transmitter and receiver channels at the CMP-input.
Mode
V
ref
25 dB speech
control
50 dB speech
control
DTMFMute
Total mute
V
CTR
0
1
2
3
(V)
Figure 13. Control modes as function of voltage applied to gain dynamics control
input CTR; I
D
=1mA.
8
ERICSSON [ ERICSSON ]
