PBL 3881
of 1.9 V. Signal at Txin will result in an
increased level at TxDet and hence also at
CMP input. The comparators will
increase the gain in the transmitter
channel and decrease it in the receiver
channel. Signal at Rxin will do vice versa.
Thus the voltages RxDet andTxDet control
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 RxDet and
TxDet 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. 8.
The decay time in the receiver and
transmitter channels is set by C2 and C3
respectively. The resistor in the time
constant is formed by an internal 100kΩ
resistor in parallel with the external
resistors R3 and R4 respectively. The
influence of eventual R1 and R2 can be
omitted.
dB
dB
Transmit
gain =
Receive
gain =
____
---------
V
= V
+
CTR
30
20
10
0
40
30
20
10
0
-10
-20
V
V
=V - 0.6 V
+
CTR
-VREF
mV
CMP
20
40
60
-60
-40
-20
0
Figure 15. Transmit and receive gain as a function of VCMP and VRef
.
The text above describes the case
when only one channel is open at a time
and there is a distinctive pause between
signals at receiver and transmitter
Txdet
Rxdet
channel inputs so the circuit will have
time to reach its idle state. See fig.16 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 input sees it, will be shorter than in
the first case. See fig. 16 F) to G). The
capacitor C1 at CMP - input sets the
speed of the gain change in the transmit-
ter and receiver channels. The capacitors
C2 and C3 should be dimensioned for a
charging time of 0.5 - 10 ms and for a
discharge time of 150 - 300 ms. The
question of switching times being a
highly subjective proposition, is in large
dependent of the language being
A
B
E
Full transmit level
D
F
G
C
CMP
Full receive level
Figure 16. Timing of the transmitter and receiver channels at the CMP-input.
Mode
spoken in the system, caused by the
varying sound pressure picture of the
different languagues. The total external
resistance being presented for the CMP -
input should be approx. 50 k (internal
balance requirement). 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 be
Full
duplex
Full speech
control
Mute
Shut
down
VCTR
Volts
0
1.0
2.0
3.0
Figure 17. Control modes as function of voltage applied to gain dynamics control
input CTR at V+ = 3.4 V.
8
ERICSSON [ ERICSSON ]
