|TEA1064AT||Low voltage versatile telephone transmission circuit with dialler interface and transmit level dynamic limiting|
|TEA1064AT Datasheet PDF : 36 Pages |
Low voltage versatile telephone transmission circuit
with dialler interface and transmit level dynamic limiting
R2, R3, R8, R9 and Zbal (see Fig.18). Maximum
compensation is obtained when the following conditions
a) R9 × R2 = R1 × (R3 + [R8//Zbal])
b) (Zbal/[Zbal + R8]) = (Zline/[Zline + R1])
If fixed values are chosen for R1, R2, R3 and R9, then
condition a) is always fulfilled provided R8//Zbal << R3.
To obtain optimum sidetone suppression, condition b) has
to be fulfilled, resulting in:
Zbal = (R8/R1) × Zline = k × Zline
where k is a scale factor; k = (R8/R1).
The scale factor k (value of R8) is chosen to meet the
• compatibility with a standard capacitor from the E6 or
E12 range for Zbal;
• Zbal//R8 << R3 to fulfil condition a) and thus ensure
correct anti-sidetone bridge operation;
• Zbal + R8 >> R9 to avoid influencing the transmit gain.
In practice Zline varies considerably with the line length and
line type. Therefore the value chosen for Zbal should be for
an average line length giving satisfactory sidetone
suppression with short and long lines. The suppression
also depends on the accuracy of the match between
Zbal and the impedance of the average line.
The line impedance for which optimum suppression is to
be obtained can be represented by
210 Ω + (1265 Ω // 140 nF). This represents a 5 km line of
0.5 mm diameter copper twisted-pair cable matched with
600 Ω (176 Ω/km; 38 nF/km).
With k = 0.64 this results in: R8 = 390 Ω;
Zbal = 130 Ω + (820 Ω // 220 nF).
The anti-sidetone network for the TEA1060 family shown
in Fig.18 attenuates the signal received from the line by
32 dB before it enters the receiving amplifier. The
attenuation is almost constant over the whole
Alternatively a conventional Wheatstone bridge can be
used as an anti-sidetone circuit (Fig.19). 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
“Versatile speech transmission ICs for electronic
telephone sets”, order number 9398 341 10011).
1. The reference used for the MUTE, DTMF and PD
inputs is SLPE.
2. A LOW level for any of these pins is defined by
connection to SLPE, a HIGH level is defined as a
voltage greater than VSLPE + 1.5 V and smaller than
VCC1 + 0.4 V.
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