PBL 386 50/2
Four-Wire to Two-Wire Gain
From (1), (2) and (3) with EL = 0:
VTR
TIPX
G4−2
=
=
TIP
I
+
L
VRX
R
R
P
F
F
Z
ZT
−
ZL
L
Z
+
TR
VTX
V
ZT
αRSN
TR
ZRX
RHP
G
+
2-4S
+ G2− 4S (ZL + 2RF + 2RP)
-
+
E
-
L
V
TX
R
R
P
-
I
L
RING
-
RINGX
Z
T
For applications where
ZT/(αRSN·G2-4S) + 2RF + 2RP is chosen to
be equal to ZL the expression for G4-2
simplifies to:
Z
RX
ZT
1
RSN
+
V
G4−2 = −
ZRX 2G2− 4S
RX
I /αRSN
L
-
Four-Wire to Four-Wire Gain
From (1), (2) and (3) with EL = 0:
VTX
PBL 386 50/2
G4−4
=
=
Figure 9. Simplified ac transmission circuit.
VRX
ZT
−
G2− 4S (ZL + 2RF + 2RP)
+ G2−4S (ZL + 2RF + 2RP )
Functional Description
and Applications
Information
ZT
αRSN
ZRX
ZT determines the SLIC TIPX to
RINGX impedance at voice
frequencies.
ZRX controls four- to two-wire gain.
VRX is the analog ground referenced
receive signal.
Hybrid Function
The hybrid function can easily be
Transmission
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
αRSN is the receive summing node current
to metallic loop current gain = 200.
General
A simplified ac model of the transmis-
sion 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
VTX
VTR
=
+ IL (2RF + 2RP )
(1)
(2)
datasheets for values on G2-4S.
G2− 4S
VTX VRX
Two-Wire Impedance
IL
+
=
To calculate ZTR, the impedance
presented to the two-wire line by the
SLIC including the fuse and protection
resistors RF and RP, let VRX = 0.
From (1) and (2):
ZT
ZRX
αRSN
cancel by letting:
VTX VRX
VTR = EL - IL · ZL
where:
(3)
+
= 0(EL = 0)
RTX
ZB
The four-wire to four-wire gain, G4-4
includes the required phase shift and
thus the balance network ZB can be
calculated from:
,
ZT
VTX is a ground referenced version of
the ac metallic voltage between the
TIPX and RINGX terminals.
ZTR
=
+ 2RF + 2RP
αRSN G2− 4S
Thus with ZTR, αRSN, G2-4S, RP and RF
known:
G2-4S is the programmable SLIC two-wire
V
RX
to four-wire gain (transmit
direction). See note below.
VTR is the ac metallic voltage between
tip and ring.
EL is the line open circuit ac metallic
voltage.
Z
= −R
=
B
TX
V
TX
ZT = αRSN G2− 4S (ZTR − 2RF − 2RP )
Z
T
+ G
(Z + 2R + 2R )
L F P
2−4S
Z
α
RX
RSN
G
Two-Wire to Four-Wire Gain
R
TX
Z
(Z + 2R + 2R )
L F P
T
2−4S
From (1) and (2) with VRX = 0:
IL
is the ac metallic current.
VTX
VTR
ZT / αRSN
G2− 4
=
=
When choosing RTX, make sure the
output load of the VTX terminal is >20kΩ.
RF is a fuse resistor.
RP is part of the SLIC protection
ZL is the line impedance.
ZT
αRSN G2− 4S
+ 2RF + 2RP
10
ERICSSON [ ERICSSON ]
