PBL 386 61/2
Preliminary
TIPX
TIP
I
+
L
R
F
Z
L
Z
+
G2-4S
-
TR
VTX
V
TR
RHP
+
-
E
L
V
TX
R
+
F
-
I
L
RING
-
RINGX
Z
T
Z
RX
RSN
+
V
-
RX
I
/α RSN
L
PBL 386 61/2
Figure 9. Simplified ac transmission circuit.
Functional Description and Applications
Information Transmission
General
A simplified ac model of the transmission
VRX is the analogue ground referenced Four-Wire to Two-Wire Gain
circuitsisshowninfigure9. Circuitanalysis
receive signal.
From (1), (2) and (3) with EL = 0:
yields:
VTX
α
RSN is the receive summing node
current to metallic loop current
gain. The nominal value of
VTX ZT
VTR ZRX ZT
αRSN
ZL
- G2-4S ( ZL + 2RF)
VTR
=
- IL • 2RF
IL
(1)
(2)
•
G4-2
=
=
G2-4S
VTX VRX
•
+
=
α
RSN = 400
ZT
ZRX αRSN
In applications where
Two-Wire Impedance
2RF - ZT/(αRSN • G2-4S) is chosen to be
equal to ZL, the expression for G4-2 simpli-
fies to:
VTR = IL • ZL - EL
(3)
To calculate ZTR, the impedance presented
to the two-wire line by the SLIC including
the fuse resistor RF, let VRX = 0.
where:
VTX is a ground referenced version
of the ac metallic voltage
between the TIPX and RINGX
terminals.
VTR is the ac metallic voltage
between tip and ring.
ZT
1
•
G4-2 = -
From (1) and (2):
ZT
ZRX 2 • G2-4S
ZTR
=
- 2RF
αRSN • G2-4S
Four-Wire to Four-Wire Gain
EL is the line open circuit ac metallic
voltage.
Thus with ZTR, G2-4S, αRSN, and RF known:
ZT = αRSN • G2-4S • (2RF - |ZTR|)
From (1), (2) and (3) with EL = 0:
IL
is the ac metallic current.
VTX ZT
G2-4S • ( ZL + 2RF)
VRX ZRX ZT
G4-4
=
=
•
RF is a fuse resistor.
Two-Wire to Four-Wire Gain
- G2-4S • ( ZL + 2RF)
G2-4S is the SLIC two-wire to four-
wire gain (transmit direction) with
a nominal value of -0.5.
αRSN
From (1) and (2) with VRX = 0:
VTX
VTR
ZT/αRSN
ZT
αRSN • G2-4S
ZL is the line impedance.
G2-4
=
=
ZT determines the SLIC TIPX to
RINGX impedance for signal in
the 0 - 20kHz frequency range.
ZRX controls four- to two-wire gain.
- 2RF
10
ERICSSON [ ERICSSON ]
