PBL 3852
IL
PBL 3852
15
2
V
+L
R18
R11
R18
Z
C8
R9
R11
R19
Zline
V
TO
RI
+Line
Z
in
R17
Zbal
R7
bal.
R7 R17
*
C
* To give receiver flat
frequency response
Zbal = R12 , R13 and C6
Figure 24. Sidetone balance.
Figure 25. Side tone network with complex R18.
What is balancing the side
tone ?
To understand that balancing the side
tone is needed to lower the amplitude
that reaches ones own ear of the signal
that is transmitted from ones own
microphone to the line and that enters
the receiver quite normally from the line.
That in presence of a strong side tone
signal one is disturbed by it and
signals entering the balancing system
from either direction, direct from
microphone or via the line, is clipped will
result in a very distorted signal entering
the receiver amplifier and thus the
earphone. Further, to remember that side balancing criteria.
tone is a small signal that is the difference R17 is to set the receiver gain. (can be a
of two large signals and that the distortion volume control potentiometer)
can be up to ten times the fundamental
The AC-signal at point c is now 1/10 of
the signal on the line because it was
further divided by two from point b.
(R9≈Zbal).
Hence 10 x R11 ≈R18 to satisfy the
frequency.
Note #1 These values ensure that the
frequency behaviour of the transmitter is
not influenced. With the ratio 1/10 the
influence is 1 dB, and with t ratio 1/20 it´s
0.5 dB.
Note #2 If the R9 is made low ohmic
compared with Zbal, it will load the latter
and result in a bad side tone perfomance,
again if the R9 is made high ohmic
compared with Zbal a low signal to
balance the side tone will result and make
the balancing difficult.
instinctively lowers ones voice level, but
again if the balance is too good (seldom
the case) the earphone will feel ”dead”.
In practical terms, what is expected, is
the same amplitude of ones own voice in
the ear as without the hand- set. The
need to lower the side tone level where
no balancing has been done is in the
order of one power of ten (20 dB).
To understand the principle both
theoretically and practically. See text.
Be sure to understand the balance that
is influenced by outer factors like, the
impedance of the line and the signal that
enters the ear acoustically directly from
the mouth through the handset. The
signal that enters the microphone from
the earphone acoustically will also
influence the return loss.
To understand the signal treatment
that is at hand. In other words that the
side tone network can be trimmed to
form a veritable ”distortion analyser”, so
that all the distortion that is present from
the microphone even if it is small, will be
the only signal entering the earphone.
This will sound very bad. It is better to
induce some of the fundamental
frequency back by making the balance
less perfect at that frequency. This is
valid for a network that is trimmed to only
one frequency. It is to strive to trim the
network such that it will at all line
A short guidance for under-
standing the side tone
principle (see fig. 26)
Assume the line impedance to be 600Ω.
Z1 = Line impedance
Z2 = The telephone set impedance 600Ω
Z1//Z2 = 300Ω
R7 will have a certain value 39 - 100Ω to
give the telephone a specified DC-
characteristic.
Assuming this DC-characteristic will
require R7 = 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 R7.
Note that the signals at points a and b are
180 degrees off phase.
Making any of the impedances
unnecessary high will make the circuit
sensitive to RFI. All values given here are
approximate and serve as starting entities
only. The final trimming of side tone
network is a cut and try proposition
because a part of the balance lies in the
accoustical path between the microphone
and earphone.
10 x R7 ≈ R9 + Zbal
R9 ≈ Zbal
note #1
note #2
Telephone
Line side
set side
a
1
17
PBL 3852
Tx
b
Rx
16
2
15
Z1
Z2
R9
R11
c
R18
R12
Zbal
R13
R17
R7
C6
combinations attenuate the harmonics
the same as the fundamental frequency.
To understand that if one of the two
Figure 26. The side tone suppression principle.
17
ERICSSON [ ERICSSON ]
