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TEA1067 Datasheet(PDF) 7 Page - NXP Semiconductors |
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TEA1067 Datasheet(HTML) 7 Page - NXP Semiconductors |
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7 / 28 page  June 1990 7 Philips Semiconductors Product specification Low voltage versatile telephone transmission circuit with dialler interface TEA1067 Automatic gain control input (AGC) Automatic line loss compensation is achieved by connecting a resistor (R6) between AGC and VEE. The automatic gain control varies the gain of the microphone amplifier and the receiving amplifier in accordance with the DC line current. The control range is 5.9 dB. This corresponds to a line length of 5 km for a 0.5 mm diameter copper twisted-pair cable with a DC resistance of 176 Ω/km and an average attenuation 1.2 dB/km. Resistor R6 should be chosen in accordance with the exchange supply voltage and its feeding bridge resistance (see Fig.13 and Table 1). The ratio of start and stop currents of the AGC curve is independent of the value of R6. If no automatic line loss compensation is required the AGC may be left open-circuit. The amplifiers, in this condition, will give their maximum specified gain. Power-down input (PD) During pulse dialling or register recall (timed loop break) the telephone line is interrupted. During these interruptions the telephone line provides no power for the transmission circuit or circuits supplied by VCC. The charge held on C1 will bridge these gaps. This bridging is made easier by a HIGH level on the PD input which reduces the typical supply current from 1 mA to 55 µA and switches off the voltage regulator preventing discharge through LN. When PD is HIGH the capacitor at REG is disconnected with the effect that the voltage stabilizer will have no switch-on delay after line interruptions. This minimizes the contribution of the IC to the current waveform during pulse dialling or register recall. When this facility is not required PD may be left open-circuit. Side-tone suppression The anti-sidetone network, R1//Zline, R2, R3, R9 and Zbal, (see Fig.5) suppresses transmitted signal in the earpiece. Compensation is maximum when the following conditions are fulfilled: (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) will always be fulfilled when R8//Zbal << R3. To obtain optimum side-tone 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), dependent on the value of R8, is chosen to meet the following criteria: (a) Compatibility with a standard capacitor from the E6 or E12 range for Zbal (b) Zbal//R8 << R3 to fulfil condition (a) and thus ensuring correct anti-sidetone bridge operation (c) Zbal + R8 >> R9 to avoid influencing the transmitter gain In practice 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. |
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