PBL 386 11/2
TIPX
TIP
I
+
L
R
F
Z
L
Z
+
TR
VTX
V
TR
RHP
G2-4S
+
-
-
E
L
V
TX
R
+
F
-
I
L
RING
-
RINGX
Z
T
Z
RX
RSN
+
V
-
RX
I
/α RSN
L
PBL 386 11/2
Figure 9. Simplified ac transmission circuit.
Functional Description and Applications
Information Transmission
General
ZRX controls four- to two-wire gain.
VRX is the analogue ground referenced
receive signal.
Four-Wire to Two-Wire Gain
A simplified ac model of the transmission
circuitsisshowninfigure9. Circuitanalysis
From (1), (2) and (3) with EL = 0:
αRSN 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)
yields:
VTX
•
G4-2
=
=
•
VTR
=
- IL • 2RF
IL
(1)
(2)
G2-4S
VTX VRX
α
RSN = 400
+
=
In applications where
ZT
ZRX αRSN
2RF - ZT/(αRSN • G2-4S) is chosen to be
equal to ZL, the expression for G4-2 simpli-
fies to:
Two-Wire Impedance
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:
ZT
1
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.
•
G4-2 = -
ZRX 2 • G2-4S
From (1) and (2):
ZT
ZTR
=
- 2RF
αRSN • G2-4S
Four-Wire to Four-Wire Gain
From (1), (2) and (3) with EL = 0:
Thus with ZTR, G2-4S, αRSN, and RF known:
ZT = αRSN • G2-4S • (2RF - |ZTR|)
EL is the line open circuit ac metallic
voltage.
VTX ZT
G2-4S • ( ZL + 2RF)
•
G4-4
=
=
IL
is the ac metallic current.
VRX ZRX ZT
- G2-4S • ( ZL + 2RF)
RF is a fuse resistor.
αRSN
Two-Wire to Four-Wire Gain
G2-4S is the SLIC two-wire to four-
wire gain (transmit direction) with
a nominal value of -0.5.
ZL is the line impedance.
ZT determines the SLIC TIPX to
RINGX impedance for signal in
the 0 - 20kHz frequency range.
From (1) and (2) with VRX = 0:
VTX
VTR
ZT/αRSN
ZT
αRSN • G2-4S
G2-4
=
=
- 2RF
10