PBL 388 14
Thecomparatorwillincreasethegaininthe
transmitter channel and decrease it in the
receiver channel accordingly. Signal at
dB
dB
Transmit
____
Receive
gain =
Rx will do the same but vice versa. The
in
gain =
---------
voltages Rx control
Tx
Detout and Detout
thus the gain setting in respective channel
through the comparator using the CMP
input as a summing point. The attack and
30
20
10
0
40
30
20
10
0
decay times for the signals Rx
Detout and
Tx
are controlled by individual
external RC-networks. The attack time in
the receiver channel is set by C together
Detout
2
V
CTR=V+
VCTR=V+
with C and by the maximum current capa-
1
bility of the detector output. The time cons-
tant is altered best by altering the value of
C
Thetransmitterchannelworkslikewise.
2.
See fig. 5.
The decay time in the receiver and
transmitter channels is set by C and C
-10
2
3
VCTR=open
VCTR=open
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.
-20
V
-VREF
mV
CMP
20
40
60
-60
-40
-20
0
Figure 11. Transmit and receive gain as a function of VCMP and VCTR
.
Rxdet
Txdet
See fig.12 F) to G). The capacitor C at
1
CMP - input sets the speed of the gain
change in the transmitter and receiver
A
B
E
F
Full recieve level
channels.ThecapacitorsC andC should
2 3
G
D
C
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
largepartdependentofthelanguagebeing
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
CMP
Full transmit level
Figure 12. Timing of the transmitter and receiver channels at the CMP-input.
Mode
detectorssensethesignalsafterF andF
2
5
respectively(F andF areaffected bythe
2
5
gain setting). For example: If the transmit-
ter channel is open (maximum gain), a
signaltokeepthetransmitterchannelopen
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 wherebygainandsensitivity
is high at small signals. There is a break
pointinthecurveatalevelof± 200mVfrom
theinternalreferencevoltage(≈2V), where
the sensitivity for increasing input signals
Vref
25 dB speech
control
50 dB speech
control
DTMF mute
Total mute
V
CTR
(V)
0
1
2
3
Figure 13. Control modes as function of voltage applied to gain dynamics control
input CTR ID=1mA
8
ERICSSON [ ERICSSON ]
