PBL 387 73
TIPX
HP
TIP
I
+
L
R
F
Z
L
Z
+
TR
VTX
V
TR
RHP
G2-4S
+
-
+
E
L
V
TX
R
F
-
-
I
L
RING
-
Z
T
RINGX
Z
RX
RSN
+
-
V
RX
I /αRSN
L
PBL 387 73
Figure 9. Simplified AC model of PBL 387 73/1.
Two-Wire Impedance
Transmission
General
To calculate ZTR, the impedance presented to the
two-wire line by the SLIC including the line protection
resistors RF , let VRX =0.
A simplified ac model of the transmission circuit is shown in
figure 9. Circuit analysis yields:
From (1) and (2):
VTX
VTR
=
+ IL × 2RF
(1)
(2)
G2-4S
ZT
ZTR
=
+ 2RF
(4)
(5)
IL
VTX VRX
αRSN ZT
α
RSN × G2-4S
=
+
ZRX
Thus with ZTR, G2-4S, αRSN and RF known:
ZT = αRSN × G2-4S × (ZTR - 2RF)
VTR = EL - IL × ZL
(3)
where:
Two-Wire to Four-Wire Gain
VTX
VTR
EL
is the ground referenced ac voltage at the VTX terminal.
is the ac metallic voltage between tip and ring.
is the line open circuit ac metallic voltage.
is the ac metallic current.
From (1) and (2) with VRX =0:
VTX
VTR
ZT/αRSN
ZT
RSN × G2-4S
(6)
G2-4
=
=
IL
+2RF
α
RF
is a line over voltage protection resistor.
G2-4S
is the SLIC two-wire to four-wire gain (transmit
direction) with a nominal value of 0.5.
Four-Wire to Two-Wire Gain
From (1), (2) and (3) with EL = 0:
ZL
is the total line impedance
VTR
VRX
ZT
1
ZL
(7)
ZRX
ZT
controls four- to two-wire gain.
×
×
G4-2
=
= -
ZRX G2-4S
ZT
RSN × G2-4S
determines the SLIC TIPX to RINGX ac impedance for
signals at voice frequencies.
+ Z +
2RF
L
α
VRX
is the analog ground referenced receive signal.
For applications where
αRSN
is the receive summing node current to metallic loop
current gain. αRSN = 400
ZT
+2RF = ZL
α
RSN × G2-4S
RHP
internal resistor, approx. 400 kΩ
the expression for G4-2 simplifies to:
ZT
1
(8)
×
G4-2 = -
ZRX 2 × G2-4S
EN/LZT 146 137 R1A ©Ericsson Microelectronics, December 2001
12
ERICSSON [ ERICSSON ]
