PBL 386 40/2
Four-Wire to Two-Wire Gain
From (1), (2) and (3) with EL = 0:
TIPX
TIP
+
I
L
VTR
R
R
P
F
G4−2
=
=
Z
L
VRX
Z
+
TR
VTX
V
TR
RHP
G
+
2-4S
-
ZT
−
ZL
+
E
L
V
TX
R
R
P
ZT
αRSN
-
F
ZRX
-
I
+ G2− 4S (ZL + 2RF + 2RP)
L
RING
RINGX
-
Z
T
Z
For applications where
RX
RSN
+
V
ZT/(αRSN·G2-4S) + 2RF + 2RP is chosen to
be equal to ZL the expression for G4-2
simplifies to:
RX
I /αRSN
L
-
PBL 386 40/2
ZT
1
G4−2 = −
ZRX 2G2− 4S
Figure 9. Simplified ac transmission circuit.
Four-Wire to Four-Wire Gain
From (1), (2) and (3) with EL = 0:
ZT
determines the SLIC TIPX to
RINGX impedance at voice
frequencies.
Functional Description
and Applications Infor-
mation
VTX
G4−4
=
=
VRX
ZRX controls four- to two-wire gain.
VRX is the analog ground referenced
receive signal.
ZT
−
G2− 4S (ZL + 2RF + 2RP)
+ G2−4S (ZL + 2RF + 2RP )
ZT
αRSN
ZRX
Transmission
αRSN is the receive summing node
current to metallic loop current
gain = 200.
General
Hybrid Function
A simplified ac model of the transmission
circuits is shown in figure 9. Circuit
analysis yields:
Note that the SLICs two-wire to four-wire
gain, G2-4S, is user programmable
between two fix values. Refer to the
The hybrid function can easily be
implemented utilizing the uncommitted
amplifier in conventional CODEC/filter
combinations. Please, refer to figure 10.
Via impedance ZB a current proportional
to VRX is injected into the summing node
of the combination CODEC/filter ampli-
fier. As can be seen from the expression
for the four-wire to four-wire gain a
voltage proportional to VRX is returned to
VTX. This voltage is converted by RTX to a
current flowing into the same summing
node. These currents can be made to
cancel by letting:
datasheets for values on G2-4S
.
VTX
(1)
(2)
(3)
VTR
=
+ IL (2RF + 2RP )
G2− 4S
VTX VRX
Two-Wire Impedance
IL
+
=
To calculate ZTR, the impedance pre-
sented to the two-wire line by the SLIC
including the fuse and protection
resistors RF and RP, let:
ZT
ZRX
αRSN
VTR = EL - IL · ZL
where:
VRX = 0.
From (1) and (2):
VTX is a ground referenced version of
the ac metallic voltage between the
TIPX and RINGX terminals.
G2-4S is the programmable SLIC two-wire
to four-wire gain (transmit
direction). See note below.
VTR is the ac metallic voltage between
tip and ring.
VTX VRX
ZT
+
= 0(EL = 0)
ZTR
=
+ 2RF + 2RP
RTX
ZB
αRSN G2− 4S
The four-wire to four-wire gain, G4-4,
includes the required phase shift and
thus the balance network ZB can be
calculated from:
Thus with ZTR, αRSN, G2-4S, RP and RF
known:
ZT = αRSN G2− 4S (ZTR − 2RF − 2RP )
V
RX
EL
is the line open circuit ac metallic
voltage.
Z
= −R
=
B
TX
V
TX
Two-Wire to Four-Wire Gain
Z
IL
is the ac metallic current.
T
+ G
(Z + 2R + 2R )
L F P
2−4S
From (1) and (2) with VRX = 0:
Z
α
RF is a fuse resistor.
RP is part of the SLIC protection.
RX
RSN
G
R
TX
Z
(Z + 2R + 2R )
L F P
T
2−4S
VTX
VTR
ZT / αRSN
G2− 4
=
=
ZT
αRSN G2− 4S
+ 2RF + 2RP
ZL
is the line impedance.
10
ERICSSON [ ERICSSON ]
