PBL 388 14
The comparator will increase the gain in the
transmitter channel and decrease it in the
receiver channel accordingly. Signal at
Rxin will do the same but vice versa. The
voltages RxDetout and TxDetout control
thus the gain setting in respective channel
through the comparator using the CMP
input as a summing point. 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 by the maximum current capa-
bility of the detector output. The time cons-
tant is altered best by altering the value of
C2. The transmitter channel works likewise.
See fig. 5.
The decay time in the receiver and
transmitter channels is set by C2 and C3
respectively. The resistor in the time cons-
tant is formed by an internal 100kΩ
resistor.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 channel
inputs so the circuit will have time to reach
its idle state. See fig.12 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.12 F) to G). The capacitor C1 at
CMP - input sets the speed of the gain
change in the transmitter and receiver
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 is
a highly subjective proposition. It is to a
large part dependent of the language being
spoken in the system, this because of the
varying sound pressure pattern in the diffe-
rent 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 transmit-
ter channel is open (maximum gain), a
signal to keep the transmitter channel open
is smaller than the signal that would be
needed to open the channel when the
receiver channel is open. The output swing
of the level detectors is matched for
variations in the supply voltage. The
detectors have a logarithmic rectifier
characteristic whereby gain and sensitivity
is high at small signals. There is a break
point in the curve at a level of
±
200mV from
the internal reference voltage (≈2V), where
the sensitivity for increasing input signals
dB
Transmit
gain = ____
Receive
gain = ---------
dB
30
40
20
30
V
CTR
=V+
V
CTR
=V+
10
0
20
10
-10
0
V
CTR
=open
V
CTR
=open
-20
V
CMP
-V
REF
mV
-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
DTMF mute
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 ]
