PBL 3853
Side Tone Suppression
T1
(See figure 13, 14)
Diodes
ensure the
necessary
supply to Rx
for full swing.
The side tone suppression is achieved by adding two signals V+L and VTO that are in
opposite phase at input RI. Because of the complex line impedance Zline, VTO must
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:
+ 2.9V
18
11k
PBL3853
12
13
-
+
11
R5 R7
1
1
1
=
+
+
Zline// (R1 + R2)
+
{
R6 R11
R9 R11 Rbal
}
7.5k
14
10
6
R11>> Zline // (R1 +R2)
R6 >> R5
Application for lower DC- supply using the
microphone amplifier as a shunt regulator.
Zbal =R8,R9,and C6
C10 is omitted in the equation
pin 18
1
Zbal
=
DC
R5 R11
R6R7 Zline // (R1 +R2)
1
1 1
R6 R7
R1
R2
R3
R1 = 11k
R2 = 22k
R3C = 4.7k
1DZ = 1000µF
1
DZ2 = 2.4V
TR1 = BC 178
TR2 = BC 547
TR1
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
does influence the transmitter gain and frequency response. (Zbal + R6 >>R5)
R7 should not be low compared with Zbal this will influence the receiver frequency
response. (R7>>Zbal)
+
= Ref 25Z
TR2
C1
DZ2
DZ1
The side tone network impedance, parallel with the receiver input impedance Zin,
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).
Shunt regulator for DC- output
Maximum compensation without any assumption is obtained when following
condition is fulfilled:
pin 18
R5
Zline // (R1 + R2)
1
1
1
R5 / R6
To get lower voltage at DC
than the 4.3V needed at pin 18
+
+
-
=R7
{
}
R5 + R6
Zline // (R1 + R2 + R11) R6 R7 Zbal R5 + R6
DC
In practice Zline varies with the line length and the feeding system
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 Ω (10-39Ω) because this
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.
R3
R1
R2
=
=
TR1
R3C = 4.7k
1DZ = 1000µF
1
DZ2
TR1 = BC 178
TR2 = BC 547
+
= Ref 25Z
=
TR2
C1
DZ2
DZ1
Figure 17.
Assuming that this DC-characteristic
requires R5=60Ω, 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.
Note that the signals at points a and b
are 180 degrees off phase.
Note#1 These values ensure that the
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.
A Short Guidance
for Understanding the Side
Tone Principle
(See fig.14)
Assume the line impedance to be 600Ω.
Z1 = Line impedance
Z2 = The telephone set impedance
600Ω
Z1//Z2 = 300Ω
R5 will have a certain value 39-100Ω to
give the telephone a specified DC-
characteristic and owercurrent
protection.
Note#2 If the R6 is made low ohmic
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
10 x R5 ≈ R6 + Zbal
R6 ≈ Zbal
Note#1
Note#2
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 ≈ Zbal)
Hence 10 x R7 ≈ R11 to satisfy the
balancing criteria.
balancing difficult.
R10 is to set the receiver gain. (can
also be a volume control potentiometer.)
10
ERICSSON [ ERICSSON ]
