Philips Semiconductors
Product specification
Low voltage transmission circuits with
dialler interface
Sidetone suppression
The anti-sidetone network, R1//Z
line
, R2, R3, R8, R9 and
Z
bal
, (see Fig.4) suppresses the transmitted signal in the
earpiece. Maximum compensation is obtained when the
following conditions are fulfilled:
R8
×
Z
bal
(1)
R9
×
R2
=
R1
×
R3
+
------------------------
-
R8
+
Z
bal
Z
line
Z
bal
------------------------
=
-------------------------
-
-
Z
bal
+
R8
Z
line
+
R1
(2)
E
XAMPLE
TEA1062; TEA1062A
The balance impedance Z
bal
at which the optimum
suppression is present can be calculated by:
Suppose Z
line
= 210
Ω
+ (1265
Ω//140
nF) representing a
5 km line of 0.5 mm diameter, copper, twisted-pair cable
matched to 600
Ω
(176
Ω/km;
38 nF/km).
When k = 0.64 then R8 = 390
Ω;
Z
bal
= 130
Ω
+ (820
Ω//220
nF).
The anti-sidetone network for the TEA1060 family shown
in Fig.4 attenuates the signal received from the line by
32 dB before it enters the receiving amplifier.
The attenuation is almost constant over the whole
audio-frequency range.
Figure 5 shows a conventional Wheatstone bridge
anti-sidetone circuit that can be used as an alternative.
Both bridge types can be used with either resistive or
complex set impedances. (More information on the
balancing of anti-sidetone bridges can be obtained in our
publication
“Applications Handbook for Wired telecom
systems, IC03b”,
order number 9397 750 00811.)
If fixed values are chosen for R1, R2, R3 and R9, then
condition (1) will always be fulfilled when
|R8//Z
bal
|
<< R3.
To obtain optimum sidetone suppression, condition (2) has
to be fulfilled which results in:
R8
Z
bal
=
-------
×
Z
line
=
k
×
Z
line
-
R1
R8
Where k is a scale factor; k
=
-------
-
R1
The scale factor k, dependent on the value of R8, is
chosen to meet the following criteria:
•
compatibility with a standard capacitor from the E6 or
E12 range for Z
bal
• Z
bal
//R8 << R3 fulfilling condition (a) and thus
ensuring correct anti-sidetone bridge operation
• Z
bal
+ R8 >> R9 to avoid influencing the transmit gain.
In practise Z
line
varies considerably with the line type and
length. The value chosen for Z
bal
should therefore be for
an average line length thus giving optimum setting for
short or long lines.
1997 Sep 03
7
PHILIPS [ NXP SEMICONDUCTORS ]
