PBL 386 30/2
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
+
ZT
−
ZL
2-4S
-
+
ZT
αRSN
E
ZRX
L
V
+ G2− 4S (ZL + 2RF + 2RP)
TX
R
R
P
-
F
-
I
L
RING
RINGX
-
Z
T
For applications where
Z
RX
ZT/(αRSN·G2-4S) + 2RF + 2RP is chosen to be
equaltoZL theexpressionforG4-2 simplifies
to:
RSN
+
V
RX
I
/αRSN
L
-
PBL 386 30/2
ZT
1
G4−2 = −
ZRX 2G2− 4S
Figure 8. Simplified ac transmission circuit.
Four-Wire to Four-Wire Gain
Functional Description
andApplicationsInforma-
tion
From (1), (2) and (3) with EL = 0:
VTX
G4−4
=
=
VRX
ZT
−
G2− 4S (ZL + 2RF + 2RP)
+ G2−4S (ZL + 2RF + 2RP )
ZT
αRSN
VRX is the analog ground referenced re-
ceive signal.
ZRX
Transmission
General
αRSN is the receive summing node current
to metallic loop current gain = 200.
A simplified ac model of the transmission
circuitsisshowninfigure8. Circuitanalysis
yields:
Hybrid Function
Note that the SLICs two-wire to four-wire
gain, G2-4S, is user programmable between
two fix values. Refer to the datasheets for
The hybrid function can easily be imple-
mented utilizing the uncommitted amplifier
inconventionalCODEC/filtercombinations.
Please, refer to figure 9. Via impedance ZB
a current proportional to VRX is injected into
the summing node of the combination
CODEC/filter amplifier. As can be seen
from the expression for the four-wire to
four-wire gain a voltage proportional to VRX
isreturnedtoVTX. Thisvoltageisconverted
by RTX to a current flowing into the same
summing node. These currents can be
made to cancel by letting:
values on G2-4S
.
VTX
(1)
VTR
=
+ IL (2RF + 2RP )
G2− 4S
VTX VRX
Two-Wire Impedance
IL
+
=
(2)
(3)
To calculate ZTR, the impedance presented
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.
VTX is a ground referenced version of the
acmetallicvoltagebetweentheTIPX
and RINGX terminals.
G2-4S is the programmable SLIC two-wire
to four-wire gain (transmit direction).
See note below.
From (1) and (2):
VTX VRX
ZT
+
= 0(EL = 0)
ZTR
=
+ 2RF + 2RP
RTX
ZB
αRSN G2− 4S
The four-wire to four-wire gain, G4-4, in-
cludes the required phase shift and thus
the balance network ZB can be calculated
from:
ThuswithZTR,αRSN,G2-4S,RP andRF known:
ZT = αRSN G2− 4S (ZTR − 2RF − 2RP )
VTR is the ac metallic voltage between
tip and ring.
V
EL
is the line open circuit ac metallic
voltage.
RX
Z
= −R
=
B
TX
V
TX
Two-Wire to Four-Wire Gain
Z
IL
is the ac metallic current.
is a fuse resistor.
is part of the SLIC protection
is the line impedance.
determines the SLIC TIPX to RINGX
impedance at voice frequencies.
T
From (1) and (2) with VRX = 0:
+ G
(Z + 2R + 2R )
L F P
2−4S
RF
RP
ZL
ZT
Z
α
RX
RSN
G
R
TX
Z
(Z + 2R + 2R )
2−4S L F P
T
VTX
VTR
ZT / αRSN
G2− 4
=
=
ZT
αRSN G2− 4S
+ 2RF + 2RP
ZRX controls four- to two-wire gain.
10
ERICSSON [ ERICSSON ]
