pbl3853 ETC-unknow, pbl3853 Datasheet - Page 10

• Manufacturer Part Number: pbl3853
• Description: Universal Speech Circuit
• Manufacturer: ETC-unknow

PBL 3853
Side Tone Suppression
(See figure 13, 14)
The side tone suppression is achieved by adding two signals V+L and V
opposite phase at input RI. Because of the complex line impedance Zline, V
be compensated by Zbal in order to get the correct level and phase for the signal to
the summing point. Maximum compensation is achieved when following conditions
are fulfilled:
R
R
R
R
Z
C
Z
does influence the transmitter gain and frequency response. (Zbal + R6 >>R5)
response. (R7>>Zbal)
should not be too high compared with Zin this influencing the spread in the receiver
gain. Zin >> side tone network impedance, R11//R10// (R7+R6//Zbal).
condition is fulfilled:
paratmeters. Therefore Zbal should be choosen to give a satisfactory side tone
suppression at an average line length. An other method is to make R11 complex. See
figure 13. This will be advantageous in case the R5 is low
coupling will give +6dB more signal for the side tone balancing. Warning! At low
values of R5 the circuit will have an insufficient overcurrent protection. A over voltage
protection with lower limiting level has to be used across the circuit. It also will make it
possible to implement a better working volume control for the earphone. There will be
some disadvantages as: More difficult to trim and need of closer tolerance
components.
A Short Guidance
for Understanding the Side
Tone Principle
(See fig.14)
Assume the line impedance to be 600 .
Z1 = Line impedance
Z2 = The telephone set impedance
600
Z1//Z2 = 300
R5 will have a certain value 39-100 to
give the telephone a specified DC-
characteristic and owercurrent
protection.
10
R
bal
bal
5
6
11
6
10
Following should be noted when designing the side tone network:
The side tone network impedance in parallel with the R5 should not be too low. This
R7 should not be low compared with Zbal this will influence the receiver frequency
The side tone network impedance, parallel with the receiver input impedance Zin,
Maximum compensation without any assumption is obtained when following
In practice Zline varies with the line length and the feeding system
R
5
=
>> R
>> Z
is omitted in the equation
=
+ R
=R
5
R
R
11
7
8
R
6
line
,R
5
R
=R
Z
5
6
line
9
R
R
// (R
,and C
7
11
7
// (R
Z
Z
1
l
line
ine
Z
+R
1
line
6
// (R
+ R
// (R
1
2
// (R
)
2
1
)
1
{
+ R
1
1
1 + 1 + 1
R
+R
+ R
9
2
2
+ R
R
2
)
)
11
11
R
)
bal
R
{
1 1
6
}
R
1
R
6
7
+
1
R
7
requires R5=60 , hence it will be 1/5 of
the Z1//Z2. This will also give 1/5 of the
ac-signal that is on the line across R5.
are 180 degrees off phase.
10 x R5
R6
The ac-signal at point c is now 1/10 of
the signal on the line because it is fur-
ther divided by two from point b.
(R6
Hence 10 x R7
balancing criteria.
also be a volume control potentiometer.)
+
Assuming that this DC-characteristic
Note that the signals at points a and b
R10 is to set the receiver gain. (can
1
Z
bal
Zbal
Zbal)
-
R
R
R6 + Zbal
5
5
+ R
/ R
6
(10-39 ) because this
6
}
R11 to satisfy the
TO
Note#1
Note#2
that are in
TO
must
Note#1 These values ensure that the
Note#2 If the R6 is made low ohmic
Figure 17.
R2
R1
11k
7.5k
Shunt regulator for DC- output
R3
12
13
frequency behaviour of the tras-
mitter is not influenced. With the
ratio 1/10 the influence is 1 dB,
and with ratio 1/20 it is 0.5 dB.
compared with Zbal, it will load
the latter and result in a bad side
tone performance, again if the
R6 is made high ohmic com-
pared with Zbal will result in a
low signal to balance the side
tone with and make the
balancing difficult.
R3
Application for lower DC- supply using the
microphone amplifier as a shunt regulator.
10
+
-
TR2
PBL3853
TR2
pin 18
DZ1
pin 18
DZ1
6
14
TR1
TR1
DZ2
To get lower voltage at DC
than the 4.3V needed at pin 18
DZ2
18
11
+
+
C1
C1
DC
R1
R2
R3C
1DZ
1
DZ2
TR1
TR2
DC
T1
R1
R2
R3C
1DZ
1
DZ2
TR1
TR2
Diodes
ensure the
necessary
supply to Rx
for full swing.
=
=
= 4.7k
= 1000 F
= Ref 25Z
=
= BC 178
= BC 547
+
= 11k
= 22k
= 4.7k
= 1000 F
= Ref 25Z
= 2.4V
= BC 178
= BC 547
+ 2.9V