pbl38571 ETC-unknow, pbl38571 Datasheet - Page 5

• Manufacturer Part Number: pbl38571
• Description: High Current Speech Line Interface Circuit
• Manufacturer: ETC-unknow

Functional description
Design procedure; ref. to fig.4.
The design is made easier through that all
settable parameters are returned to gro-
und (-line), this feature differs
bridge type solutions.To set the parameters
in the following order will result in that the
interaction between the same is minimized.
either resistive (600 ) or complex. (R3
and C1). C1 should be big enough to give
low impedance compared with R3 in the
telephone speech frequency band.Too
large C1 will make the start-up slow. See
fig. 6.
required in the PTT specification or in
case of a system telephone,in the PBX
specification (R6).There are also internal
circuit dependent requirements like supply
voltages etc.
length regulation is supposed to cut in
(R1 and R2 in fig. 18). Note that in some
countries the line length regulation is not
allowed. In most cases the end result is
better and more readily achieved by using
the line length regulation (line loss
compensation) than without. See fig. 13.
frequency response.
response. See text how to limit the max.
swing to the earphone.
network.
components in case necessary. In two
piece telephones the often ”helically”
wound
microphone input with its high gain is
especially sensitive.
other protection devices used in the de-
sign, a good practice is to connect a 15V
1W zener diode across the circuit , from
pin 1 to -Line.
Figure 7. Block connections.
1. Set the circuit impedance to the line,
2. Set the DC-characteristic that is
3. Set the attac point where the line
4.Set the transmitter gain and
5. Set the receiver gain and frequency
6. Adjust the side tone balancing
7. Set
8. Circuit protection. Apart from any
cord acts as an aerial. The
1
the RFI
+
Mute
AM
suppression
Transmitter summing
input
it from
2
AT
Impedance to the line
set by R3, C1 and C2. Fig.4. The circuits
relatively high parallel impedance will not
influence it to any noticeable extent. At low
frequencies the influence of C1 can not be
neglected. Series resistance of C1 that is
dependent on the temperature and the
quality of the component will cause some
of the line signal to enter pin 6. This
generates a closed loop in the transmitter
amplifier that in it´s turn will create an
active impedance thus lowering the
impedance to the line. The impedance at
high frequencies is set by C2 that also
acts as a RFI suppressor.
impedance towards the line is specified as
a complex network. See fig. 6. In case a).
the error signal entering pin 6 is set by the
ratio Rs/R19 (910 ), where in case b).
the ratio at high frequencies will be Rs/
220
Figure 6. AC-impedance.
C2
The AC- impedance to the line is
because the 820
4
In many specifications the
PBL 38 571
3
3
AR
+ Line
- Line
resistor is
1
2
R6
6
bypassed by a capacitor. To help up this
situation the complex network capacitor is
connected directly to ground, case c).
making the ratio Rs/220 +820 and thus
lessening the error signal. Conclusion:
Connect like in case c) when complex
impedance is specified.
DC - characteristic
telephone set has to fulfill is mainly given
by the network administrator. Following
parameters are useful to know when the
DC behaviour of the telephone is to be set:
•
•
•
•
•
•
+
a)
Rs
R3
C1
1
The voltage of the feeding system
The line feeding resistance 2 x.......
ohms.
The maximum current from the line at
zero line length.
The min. current at which the telephone
has to work (basic function).
The lowest and highest voltage
permissible across the telephone set.
The highest voltage that the
telephone may have at different line
currents.
network owners specification.The
lowest voltage for the telephone is
normally set by the voltages that are
needed for the different parts of the
telephone to function. For ex. for
transmitter output amplifier,
ver output amplifier, dialler, speech
switching.
The DC - characteristic that a
Example:
How to connect a
complex network.
220 +820 //Cx
+Line
-Line
b)
Normally set by the
PBL 385 71
220
820
c)
recei-
Cx
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