TEA1068T Philips Semiconductors, TEA1068T Datasheet - Page 6

• Manufacturer Part Number: TEA1068T
• Description: Versatile telephone transmission circuit with dialler interface
• Manufacturer: Philips Semiconductors

Philips Semiconductors
Mute input (MUTE)
A HIGH level at MUTE enables the DTMF input and
inhibits the microphone and the receiving amplifier inputs.
A LOW level or an open circuit has the reverse effect.
MUTE switching causes only negligible clicks at the
earpiece outputs and on the line.
Dual-Tone Multi Frequency input (DTMF)
When the DTMF input is enabled, dialling tones may be
sent onto the line. The voltage gain from DTMF to LN is
typically 25.5 dB (when R7
the same way as the gain of the microphone amplifier.
The signalling tones can be heard in the telephone
earpiece at a low level (confidence tone).
Receiving amplifier: IR, QR+, QR and GAR
The receiving amplifier has one input IR and two
complementary outputs, a non-inverting output QR+ and
an inverting output QR . These outputs may be used for
single-ended or for differential drive depending on the
sensitivity and type of earpiece used (see Fig.12). Gain
from IR to QR+ is typically 25 dB (when R4 = 100 k ).
This is sufficient for low-impedance magnetic or dynamic
microphones, which are suited for single-ended drive.
By using both outputs (differential drive), the gain is
increased by 6 dB. This feature can be used when the
earpiece impedance exceeds 450 , (high-impedance
dynamic or piezoelectric types).
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 between
17 dB and 33 dB with single-ended drive and between
26 dB and 39 dB with differential drive to suit the sensitivity
of the transducer used. The gain is set by the external
resistor R4 connected between GAR and QR+. Overall
receive gain between LN and QR+ is calculated by
subtracting the anti-side-tone network attenuation (32 dB)
from the amplifier gain. Two external capacitors,
C4 = 100 pF and C7 = 10
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
1996 Apr 23
Versatile telephone transmission circuit
with dialler interface
C4 = 1 nF, are necessary to
68 k ) and varies with R7 in
C4.
6
Automatic Gain Control input AGC
Automatic line loss compensation is achieved by
connecting a resistor R6 between AGC and V
automatic gain control varies the microphone amplifier
gain and the receiving amplifier gain 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 /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). 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. During these
interruptions, the telephone line provides no power for the
transmission circuit or circuits supplied by V
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 A and switches off
the voltage regulator, thus 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
Suppression of the transmitted signal in the earpiece is
obtained by the anti-side-tone network consisting of
R1//Z
compensation is obtained when the following conditions
are fulfilled:
R9 R2
Z
bal
line
, R2, R3 and Z
Z
=
bal
R1 R3
+
R8
=
+
Z
bal
line
R8//Z
(see Fig.14). Maximum
Z
bal
line
+
R1
Product specification
TEA1068
CC
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EE
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