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TEA1062T Datasheet(PDF) 7 Page - NXP Semiconductors

Part No. TEA1062T
Description  Low voltage transmission circuits with dialler interface
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Maker  PHILIPS [NXP Semiconductors]
Homepage  http://www.nxp.com
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TEA1062T Datasheet(HTML) 7 Page - NXP Semiconductors

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1997 Sep 03
7
Philips Semiconductors
Product specification
Low voltage transmission circuits with
dialler interface
TEA1062; TEA1062A
Sidetone suppression
The anti-sidetone network, R1//Zline, R2, R3, R8, R9 and
Zbal, (see Fig.4) suppresses the transmitted signal in the
earpiece. Maximum compensation is obtained when the
following conditions are fulfilled:
(1)
(2)
If fixed values are chosen for R1, R2, R3 and R9, then
condition (1) will always be fulfilled when
|R8//Zbal| << R3.
To obtain optimum sidetone suppression, condition (2) has
to be fulfilled which results in:
Where k is a scale factor;
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 Zbal
•Zbal//R8 << R3 fulfilling condition (a) and thus
ensuring correct anti-sidetone bridge operation
•Zbal + R8 >> R9 to avoid influencing the transmit gain.
In practise Zline varies considerably with the line type and
length. The value chosen for Zbal should therefore be for
an average line length thus giving optimum setting for
short or long lines.
R9
R2
×
R1
R3
R8
Z
bal
×
R8
Z
bal
+
-------------------------
+



×
=
Z
bal
Z
bal
R8
+
-------------------------
Z
line
Z
line
R1
+
--------------------------
=
Z
bal
R8
R1
--------
Z
line
kZ
line
×
=
×
=
k
R8
R1
--------
=
EXAMPLE
The balance impedance Zbal at which the optimum
suppression is present can be calculated by:
Suppose Zline = 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;
Zbal = 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.)


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