PBL 385 70
Functional description
Design procedure
; ref. to fig.4.
The design is made easier through that all settable parameters are returned to ground (-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.
1.
Set the circuit impedance to the line, either 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.
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 circuit dependent requirements like supply voltages etc.
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 endresult is better and more readily achieved by using
the line length regulation (line loss compensation) than without. See fig. 12.
Set the transmitter gain and frequency response.
Set the receiver gain and frequency response. See text how to limit the max. swing to the earphone.
Adjust the side tone balancing network.
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.
Circuit protection. Apart from any other protection devices used in the design a good practice is to connect a 15V 1W
zener diode across the circuit , from pin 1 to -Line.
2.
3.
4.
5.
6.
7.
8.
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 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 temperature and quality will
cause some of the line signal to enter pin
4. This generates 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 C2 that also
acts as a RFI suppressor.
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/
R19 (909Ω), 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
complex network capacitor is connected
directly to ground (-line), case c). making
the ratio Rs/220Ω+820Ω and thus less-
ening the error signal. Conclusion: Connect
like in case c) when complex impedance
is specified.
+Line
1
4
a)
R3
b)
c)
PBL 385 70
3
2
Rs
≈1Ω
+
C1
Example:
How to connect a
complex network.
-Line
C2
R6
Figure 6. AC-impedence
5
ERICSSON [ ERICSSON ]
