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TEA1066T

Versatile telephone transmission circuit with dialler interface

Philips Electronics 

Philips Semiconductors

Versatile telephone transmission circuit

with dialler interface

Product specification

TEA1066T

These outputs may be used for single-ended or for

differential drive, depending on the sensitivity and type of

earpiece used (see Fig.10). Gain from IR to QR+ is

typically 25 dB. This will be sufficient for low-impedance

magnetic or dynamic earpieces, which are suited for

single-ended drive. By using both outputs (differential

drive), the gain is increased by 6 dB and differential drive

becomes possible. This feature can be used when the

earpiece impedance exceeds 450 Ω (high-impedance

dynamic, magnetic or piezoelectric earpieces).

The output voltage of the receiving amplifier is specified for

continuous-wave drive. The maximum output voltage will

be higher under speech conditions, where the ratio of peak

to RMS value is higher.

The receiving amplifier gain can be adjusted over a range

of ±8 dB to suit the sensitivity of the transducer used.

The gain is set by the external resistor R4 connected

between GAR and QR+.

Two external capacitors, C4 = 100 pF and

C7 = 10 × C4 = 1 nF, are necessary to ensure stability.

A larger value of C4 may be chosen to obtain a first-order,

low-pass filter. The ‘cut-off’ frequency corresponds with

the time constant R4 × C4.

Automatic gain control input AGC

Automatic line loss compensation is obtained by

connecting a resistor R6 between AGC and VEE. This

automatic gain control varies the microphone amplifier

gain and the receiving amplifier gain in accordance with

the DC line current.

The control range is 6 dB. This corresponds with 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 of 1.2 dB/km.

Resistor R6 should be chosen in accordance with the

exchange supply voltage and its feeding bridge resistance

(see Fig.11 and Table 1). Different values of R6 give the

same ratio of line currents for start and end of the control

range.

If automatic line loss compensation is not required, AGC

may be left open. The amplifiers then all give their

maximum gain as specified.

Power-down input PD

During pulse dialling or register recall (timed loop break)

the telephone line is interrupted, as a consequence it

provides no supply for the transmission circuit and the

peripherals connected to VCC. These gaps have to be

bridged by the charge in the smoothing capacitor C1.

The requirements on this capacitor are relaxed by applying

a HIGH level to the PD input during the time of the loop

break, which reduces the supply current from typically

1 mA to typically 55 µA.

A HIGH level at PD further disconnects the capacitor at

REG, with the effect that the voltage stabilizer will have no

switch-on delay after line interruptions. This results in no

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.

Side-tone suppression

Suppression of the transmitted signal in the earpiece is

obtained by the anti-side-tone network consisting of

R1//Zline, R2, R3, R8, R9 and Zbal (see Fig.14). Maximum

compensation is obtained when the following conditions

are fulfilled:

R9 × R2 = R1 (R3 + [R8//Zbal] )

(1)

Zbal ⁄ (Zbal + R8) = Zline ⁄ (Zline + R1)

(2)

If fixed values are chosen for R1, R2, R3, and R9, then

condition (1) will always be fulfilled, provided that

R8//Zbal < R3. To obtain optimum side-tone

suppression, condition (2) has to be fulfilled, resulting in:

Zbal = (R8/R1) Zline = k × Zline, where k is a scale factor:

k = (R8/R1).

Scale factor k (dependent on the value of R8) must be

chosen to meet the following criteria:

1. Compatibility with a standard capacitor from the E6 or

E12 range for Zbal

2. Zbal//R8 << R3

3. Zbal + R8 >> R9.

In practice, Zline varies greatly with line length and cable

type; consequently, an average value has to be chosen for

Zbal. The suppression further depends on the accuracy

with which Zbal/k equals the average line impedance.

Example: The balanced line impedance Zbal at which

the optimum suppression is preset can be calculated by:

Assume 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-side-tone 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.

1996 Apr 04

6

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