PBL 385 41
Functional description
+Line
b)
Design procedure; ref. to fig.4.
The design is made easier through that all
settable parameters are returned to gro-
und(-line),thisfeaturediffersitfrombridge
typesolutions.Tosettheparametersinthe
following order will result in that the
interactionbetweenthesameisminimized.
1. Setthecircuitimpedancetotheline,
either resistive (600Ω) 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.
a)
c)
220Ω
1
2
PBL 385 41
4
R3
Cx
820Ω
3
Example:
Rs
≈1Ω
How to connect a
complex network.
220Ω+820Ω//Cx
R6
C2
+
C1
2. Set the DC-characteristic that is
requiredinthePTTspecificationorincase
of a system telephone,in the PBX
specification(R6). Observe the power
dissipated.There are also internal circuit
dependentrequirements likesupplyvolta-
ges etc.
3. Set the attac point where the line
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.
-Line
Figure 6. AC-impedance.
complex network capacitor is connected
directlytoground,casec).makingtheratio
Rs/220Ω+820Ω and thus lessening the
error signal. Conclusion: Connect like in
case c) when complex impedance is
specified.
Impedance to the line
The AC- impedance to the line is
set by R3, C1 and C2. Fig.6. The circuits
relatively high parallel impedance will not
influenceittoanynoticeableextent.At low
frequenciestheinfluenceof C1cannot 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.
DC - characteristic
4. Set the transmitter gain and
frequency response.
5. Setthereceivergainandfrequency
response. See text how to limit the max.
swing to the earphone.
The DC - characteristic that a
telephone set has to fulfill is mainly given
by the network administrator. Following
parameters are useful to know when the
DC behaviourofthetelephoneistobeset:
6. Adjust the side tone balancing
network.
7. Set the RFI suppression
components in case necessary. In two
piece telephones the often ”helically”
wound cord acts as an aerial. The
microphone input with its high gain is
especially sensitive.
8. Circuit protection. Apart from any
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.
•
•
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.Thelowestvoltageforthe
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 output amplifier, dialler,
speech switching and loudspeaker
amplifier in a handsfree telephone etc.
In many specifications the
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 (910Ω), where in case b). the
ratio at high frequencies will be Rs/220Ω
because the 820Ω resistor is bypassed by
a capacitor. To help up this situation the
•
•
•
•
+ Line
3
1
AR
AM
AT
2
+
4
Transmitter summing
input
- Line
Mute
Figure 7. Block connections.
5
ERICSSON [ ERICSSON ]
