PBL 3881
TRANSMITTER
OUTPUT
14
POWER
AMP. IN
R
CTR
PBL 3881
Rx
Tx
out
out
5
10
16
C
C
+L
Control
–
+
+
–
P1
F3
F2
F6
F5
R
15
GND
+
C
RECEIVER
INPUT
R
R
4
3
1
F2
in
C
11
13
C
F4out (AGC)
Rx
+
R
F1
out
C
in
Tx
C
F4
in
Ref.
F1
12
2
Rx
C
C
C
in
Tx
7
6
9
8
in
N
Tx
CMP
Rx
Det
R6
Det
+
Det
R
R1
R2
R4
220µF
R5
R3
Mic.
C
+
470k
+
C4
C2
C3
C1
Background noise detector
quick start
Figure 18. Speech switching arrangement.
necessary 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
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 ± 200 mV from the
internal reference voltage 1.9 V, where
the sensitivity for increasing input signals
decreases by a factor of 10, increasing
the detectors dynamic range.
Mode
Full
duplex
Full speech
control
I
CTR
-40
-20
20
µA
Mute
See fig. 12.
Background Noise Detector
Figure 19. Control modes as function of current applied to gain dynamics control
input CTR pin.
The general function of the background
noise detector in the transmittng channel
is to create a negative signal (in respect
to the reference) so that, when coupled
to the summing point at the CMP input,
will counteract the signal from the
transmitter level detector representing
the actual sound pressure level at the
microphone. This counteracts the noise
from influencing the switching characte-
ristics. The input signal to the backround
noise level detector is taken from the
output of the transmitter detector, a
voltage representing the envelope of the
amplified microphone signal. The
27K
0.1mA
V
( 3.2V pin 15)
250K
22 K
(pin 14)
CTR
Figure 20. The control input voltage arrangement, from full duplex to full
speech control.
detector inverts and amplifies this signal
2 x (transmitting mode) and has on it´s
9
ERICSSON [ ERICSSON ]
