Versatile telephone transmission circuit with
The maximum hook-off current then becomes:
Ihook-off = VZ/R1 + VBE ×
(R9a + R9b + R1)/(R1 × R9b)
where VZ is the Zener voltage of diode D5 (see Fig.18).
Suppression of the transmitted signal in the earpiece is
obtained by the anti-sidetone network comprising R1//Zline,
R2, R3, R9 and Zbal (see Fig.18). Maximum compensation
is obtained when the following conditions are fulfilled:
a) R9 × R2 = R1 × (R3 + R8)
b) k = R3 × (R8 + R9)/(R2 × R9)
c) Zbal = k × Zline
The scale factor k is chosen to meet the compatibility with
a standard capacitor from the E6 or E12 range for Zbal.
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 long and short times. The suppression
also depends on the accuracy of the match between
Zbal and the impedance of the average line.
With k = 1, R1 = 619 Ω, R9 = 20 Ω and an average line
impedance represented by 270 Ω + (120 nF // 1100 Ω), the
calculation results in:
• R2 = 130 kΩ
• R3 = 3650 Ω
• R8 = 715 Ω
The anti-sidetone network for the TEA1060 family, shown
in Fig.15, attenuates the signal received from the line by
32 dB before it enters the receiving amplifier. The
attenuation is almost constant over the whole
More information on the balancing of the anti-sidetone
bridges can be obtained in our publication “Versatile
speech transmission ICs for electronic telephone sets”,
order number 9398 341 10011.
handbook, full pagewidth
hook-off speech mode pulse dialling mode
Fig.14 Example of line current shape in pulse dialling mode (see also Fig.18).