PBL38541 Ericsson, PBL38541 Datasheet - Page 5

• Manufacturer Part Number: PBL38541
• Description: Universal Speech Circuit
• Manufacturer: Ericsson

Figure 7. Block connections.
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 it from 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). Observe the power
dissipated.There are also internal circuit
dependent requirements like supply volta-
ges etc.
length regulation is supposed to cut in
(R1 and R2). 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.
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
Transmitter summing
suppression
input
2
AT
Impedance to the line
set by R3, C1 and C2. Fig.6. 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 4. 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 4 is set by the
ratio Rs/R3 (910 ), where in case b). the
ratio at high frequencies will be Rs/220
because the 820 resistor is bypassed by
a capacitor. To help up this situation the
Figure 6. AC-impedance.
4
The AC- impedance to the line is
In many specifications the
3
C2
PBL 385 41
AR
3
+ Line
- Line
1
2
R6
4
complex network capacitor is connected
directly to ground, case c). making the ratio
Rs/220 +820
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)
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.
Normally set by the network owners
specification.The lowest voltage for the
telephone is normally set by the volta-
ges that are needed for the different
parts of the telephone to function. For
ex. for transmitter output amplifier,
receiver
speech switching and
amplifier in a handsfree telephone etc.
Rs
R3
C1
1
The DC - characteristic that a
Example:
How to connect a
complex network.
220 +820 //Cx
+Line
-Line
b)
output amplifier, dialler,
and thus lessening the
PBL 385 41
820
220
c)
loudspeaker
Cx
5