PBL 3853
Functional Description
Design Procedure
The first decision to make is, how much
current is needed at what VDC and how
much line current is available at longest
line length.
1. Set the circuit impedance to the line,
either active or passive. C
3
should be
big enough to give low impedance
compared with R1 in the telephone
speech frequency band. Too large C
3
will make the ”start up” slow.
2. Set the DC-characteristic that is
required in the PTT specification, or in
case of a system telephone design, in
the PABX specification (R5).
3. If the line length regulation (line loss
compensation) is used, set the attac
point where it should start (RC and
RD). Using the line length regulation
makes it in most cases easier to
achieve the gain/line length mask in
both transmitter and receive function.
Note, that in some countries the line
length regulation is not allowed.
4. Set the transmitter gain and
frequency response. See text for the
clipping feature.
5. Set the receiver gain and frequency
response.
6. Adjust the side tone balancing
network.
7. Apply the RFI suppression
components in case necessary. In two
piece telephones the often ”helically”
wound cord acts as an aerial where
especially the microphone input with
its high gain and input impedance is
the more sensitive.
Impedance to the Line
The AC-impedance to the line is set by
R1 (+ R2 if active impedance is used)
and C
2
. See figure 4. The circuits
relatively high parallel impedance will
influence it to some extent. At low
frequencies the influence of the C
3
can
not be neglected. Series resistance of
the C
3
that is dependent on temperature
and quality will cause that some of the
line signal will enter pin 4 and generate a
closed loop in the transmitter amplifier
that will create an active impedance thus
lowering the impedance to the line. The
impedance at high frequencies is set by
C
2
that also acts as a RFI supressor.
In many specifications the R1 is
specified as a complex network. See
figure 6 b) in the example. In case a) the
error signal entering pin 4 is set by the
ratio
≈
RS/R1 (909
Ω
Swedish spec.),
where in case b) the ratio at high
frequency will be RS/220 because the
820
Ω
resistor is 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 influence of the error signal. To save
current the circuit can be implemented
to have an active impedance to the line,
the level is set by resistors R1 and R2.
When an active impedance is used the
transmitter (see figure 16) amplifier does
not feel its own active output-impedance
thus using less current to create output
swing to the line. Case c) above can not
be used together with active impedance.
Do not use the active impedance if not
necessary, it complicates things greatly.
A full mathematical expression is
found under Detailed Description.
DC - Characteristics
The DC - characteristic that a telephone
set has to fulfill is mainly given by the
network administrator. Following para-
meters are useful to know when the DC
behaviour of the telephone is to be set:
• The voltage of the feeding system
• The line feeding resistance 2 x
Ω
• The maximum current from the line at
zero line length
• The minimum current at which the
telephone has to work (basic
function)
• The lowest and highest voltage
across the telephone
The DC-characteristic of the circuit is
a function of the voltage on pin 4. There
is also a possibility to adjust the DC-
characteristic with resistors (dc-voltage)
at pin 5 (RA and RB in figure 4). Note
that altering the DC-characteristic slope
will also influence the line length
regulation (when used) and thus the gain
of both transmitter and receiver. A closer
mathematical study is done under
Detailed Description.
Line Length Regulation
The line length regulation is to
compensate the gain in both transmitter
and receiver with changing line length
(impedance). The dynamic range of
regulation is
≈
6 dB. The starting point of
the regulation can be set by RC and RD
that take the information from the circuits
supply voltage which actually mirrors the
line current value in voltage. In case line
length regulation is not required it can be
omitted either in the high or in the low
gain mode (6 dB range of regulation).
a) Real impedance
b) Complex impedance
c)
b)
a)
R1
+Line
C2
R1
R1
C2
+Line
PBL 3853
Circuit supply
I=0.3mA
V
F
PBL3853
4
Example: b)
A complex network
220Ω + 820Ω//115nF
The complex network should
be connected to the speech
circuit like shown in c). See text.
Rs
≈1Ω
C3
Circuit supply
I=0.3mA
4
V
F
C3
The voltage across the circuit can be increased
by method shown above without influencing the
impedance towards the line.
Figure 6. AC-impedance, to the line.
Figure 7. Adjusting voltage level across
the circuits.
6
ERICSSON [ ERICSSON ]
