PBL 3853
capacitor to ground if the input tends to
pick up noise. This input can also be
used as a second input to the transmitter
(in-impedance approx. 8 kΩ, gain
depending on the arrangement at pin 3)
used for ex. DTMF tones.
across 300 Ω load (600 Ω line parallel
with 600 Ω output impedance of the
circuitry) plus 0.8 mA to VDC and pin 18
in order to get 0.6 Vp across 350 Ω
earphone (according to the test circuit)
the sum is 8.6 mA line current, or for
0.4 Vp across 150 Ω earphone the sum
being 8.8 mA line current. The rest of
the available line current can be utilized,
by decreasing R13, to charge VDC. In
this manner simplified circuit should be
adapted to the requirement for the VDC,
both in voltage and current, related to
under which line conditions the circuit
has to work. The circuit gets more
complex the closer the operational limits
are utilized. The drawback in using the
active impedance towards the line, is
the difficulty of dimensioning (especially
in case of complex line impedance
requirement) that limits its usage to only
when absolutely necessary.
so called ”softclipping” is a methode
where the line voltage is monitored and
the transmitter gain is controlled in case
the line voltage gets to high. This can be
adapted to PBL 3853 as follows. The line
length regulation function is used to
control the gain down when high signals
appear on the line, no matter if line length
regulation is desired or not. In the case
the function is used there is the high gain
situation at a long line (where it is actual
to regulate the gain down) but in case the
function is not used the transmitter is set
to have the high gain by making the RD
enough low ohmic and omitting RC.
The 1k resistor is to prevent an
If there is with passive impedance still
extra margin in current (more that 4 mA
+ 1.3 mA = 5.3 mA) then it is possible to
increase the current through R12 versus
current through R13 by making the R12
go towards 0. The whole transmitting
current would go through pin 1 to the line
and the charge current for VDC would
be set by a diode voltage drop across
R13 which would make the current that
can be taken out from VDC constant,
regardless the line current. (assuming
that the line current is high enough) The
lowest current for such a circuit is, if no
more current is taken out of the VDC
than what is needed for the earphone
amplifier: 540 mA into pin 4 plus 600 mA
supply current into pin 1 plus 6.7 mA
transmitting current to give 2 Vp signal
unlinear load to the line and also to form
the ”on” time constant of the function with
the capacitor CB. A capacitor of
0.1 µF will block the DC from the line but
will also set the lower frequency
characteristic for the function. The two
diodes form a voltage doubler rectifier for
the AC signal which is then filtered by
CB, the resistor of 10M will set the ”off”
time constant. The transistor acts as a
impedance transformer. Resistor RE sets
the level at which the amplitude limiter
cuts in. It is possible to connect diodes in
series with the pin 6 (pointing to the pin)
in order to alter the ”edge” of the limiter.
The value of RE is set by the values of
RC and RD. A component value sugges-
tion for the case of no gain regulation
with line length:
Softclipping
(see Fig.22)
The risk for signal clipping increases on
long lines especially when using the line
length regulation that doubles the gain
on such a line. This clipping is highly
uncomfortable for the second party but
also for one self because it destroys the
side tone balance and thus allows
shockwise 20 dB higher levels into the
receiver of ones own voice. There is a
cure for this especially if an active
18
CA
4
+line
0.1µF
RC
1k
R
E
6
C
22n
B
10M
R
D
14
-line
"Softclipping" for PBL 3853, rest as in Fig 4.
impedance is used towards the line. A
RC = RD = 8.2k RE = 4.7k.
Figure 22.
+5V
+ Line
+
R13
R12
+12V
9
1
R1
R2
IC 1
IC2
IC4
PBL 3853
C15
IL300
-
LM358
C1
68nF
7
5
IC 5a
T2
1
2
AC-DC char.adjust
+
8
3
2
+
TX
4
3
+
-
1/2
4
8
1
T1
10
-
R28
464
+
-
Tx mute
C13
D4
D3
D1
6
5
4
6
5
3
4
18
+V
100pF
12
D1
D2
Line in
16
C10
D1-D4
R23
R30
33k
13
6
schottky
330k
14
11
3
2
17
15
GND
-12V
6
5
C7
C5
C8
R6
R7
IC3
IL300
R29
150k
Rc
R11
C3
R8
R9
3
6
5
4
3
C21
100pF
R5
R4
R3
R10
C2
C6
R
D
IC 5b
-Line
6
-
7
4
2
1/2
RX
+
5
C14
68nF
Current direction
information
R24
33k
1
Figure 23. Analog line interface to a digital PABX with galvanic insolation by linear optocouplers (IL3000) using PBL 3853.
14
ERICSSON [ ERICSSON ]
