PBL 386 65/2
10
Preliminary
V
RX
is the analogue ground referenced
receive signal.
α
RSN
is the receive summing node
current to metallic loop current
gain. The nominal value of
α
RSN
=400
Two-Wire Impedance
To calculate Z
, the impedance presented
to the two-wire line by the SLIC including
the fuse resistor R
F
, let V
RX
= 0.
From (1) and (2):
Z
TR
= Z
T
- 2R
F
α
RSN
G
2-4S
Thus with Z
TR
, G
2-4S
,
α
RSN
, and R
F
known:
Z
T
=
α
RSN
G
2-4S
(2R
F
- |Z
TR
|)
Two-Wire to Four-Wire Gain
From (1) and (2) with V
RX
= 0:
2-4
= V
TX
= Z
T
/
α
RSN
G
TR
Z
T
F
α
RSN
- 2R
2-4S
Four-Wire to Two-Wire Gain
From (1), (2) and (3) with E
L
= 0:
G
4-2
= V
TR
= Z
T
Z
L
V
RX
Z
RX
Z
T
- G
2-4S
( Z
L
+ 2R
F
)
α
RSN
In applications where
2R
- Z
T
/(
α
G
) is chosen to be
equal to Z
L
, the expression for G
4-2
simpli-
fies to:
G
4-2
= - Z
T
1
Z
RX
2
G
2-4S
Four-Wire to Four-Wire Gain
From (1), (2) and (3) with E
L
= 0:
4-4
= V
TX
Z
T
G
2-4S
( Z
L
+ 2R
F
)
G
RX
RX
Z
T
- G
2-4S
( Z
L
+ 2R
F
)
α
RSN
Figure 9. Simplified ac transmission circuit.
PBL 386 65/2
+
-
+
-
VTX
RSN
I
L
/α
TIPX
RINGX
+
-
E
L
+
-
TIP
RING
R
F
R
F
Z
TR
Z
T
V
TX
V
RX
-
Z
RX
I
L
I
L
R
HP
+
Z
L
V
TR
G
RSN
2-4S
Functional Description and Applications
Information Transmission
General
A simplified ac model of the transmission
circuits is shown in figure 9. Circuit analysis
yields:
V
TR
= V
TX
- I
L
2R
F
G
2-4S
V
TX
+ V
RX
L
Z
T
Z
RX
α
RSN
(1)
(2)
V
TR
= I
L
Z
L
- E
L
(3)
where:
V
TX
is a ground referenced version
of the ac metallic voltage
between the TIPX and RINGX
terminals.
is the ac metallic voltage
between tip and ring.
is the line open circuit ac metallic
voltage.
is the ac metallic current.
is a fuse resistor.
G
2-4S
is the SLIC two-wire to four-
wire gain (transmit direction) with
a nominal value of -0.5.
(Phase shift 180
°
)
Z
L
is the line impedance.
Z
T
determines the SLIC TIPX to
RINGX impedance for signal in
the 0 - 20kHz frequency range.
Z
RX
controls four- to two-wire gain.
V
TR
E
L
I
L
R
F