PBL 385 71
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), this feature differs it from
bridgetypesolutions.Tosettheparameters
in the 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 38 571
6
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
required in the PTT specification or in
case of a system telephone,in the PBX
specification (R6).There are also internal
circuitdependentrequirements likesupply
voltages etc.
-Line
Figure 6. AC-impedance.
3. Set the attac point where the line
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.
4.Set the transmitter gain and
frequency response.
5. Setthereceivergainandfrequency
response. See text how to limit the max.
swing to the earphone.
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.
bypassed by a capacitor. To help up this
situation the complex network capacitor is
connected directly to ground, case c).
making the ratio 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.4. 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 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.
DC - characteristic
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:
•
•
The voltage of the feeding system
The line feeding resistance 2 x.......
ohms.
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 6 is set by the
ratio ≈Rs/R19 (910Ω), where in case b).
the ratio at high frequencies will be Rs/
220Ω because the 820Ω resistor is
•
•
•
•
The maximum current from the line at
zero line length.
Themin.currentatwhichthetelephone
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 voltages that are
needed for the different parts of the
telephone to function. For ex. for
transmitter output amplifier, recei-
ver output amplifier, dialler, speech
switching.
+ Line
3
1
AR
AM
AT
2
+
4
Transmitter summing
input
- Line
Mute
Figure 7. Block connections.
5
ERICSSON [ ERICSSON ]
