PBL38661-2 Datasheet, PDF (10/18 Pages) Ericsson – Subscriber Line Interface Circuit

PBL 386 61/2
Preliminary
+
TIP
Z_L
V_TR
Z_TR
-
E_L
-
+
RING
TIPX
R_F
R_F
RINGX
R_HP
+
-G^2-4S
I_L
VTX
I_L
Z_T
+
V_TX
-
PBL 386 61/2
Z_RX
I _L /α_RSN
RSN
+
V_RX
-
Figure 9. Simplified ac transmission circuit.
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
G_2-4S
- I_L • 2R_F
(1)
α ^{VTX + VRX = IL}
Z_T Z_RX
RSN
(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.
V_TR is the ac metallic voltage
between tip and ring.
E_L is the line open circuit ac metallic
voltage.
I_L is the ac metallic current.
R_F 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.
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_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_TR, 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):
α ^{ZTR =}
Z_T
G _RSN • _2-4S
- 2R_F
Thus with Z_TR, G_2-4S, α_RSN, and R_F known:
α Z_T = _RSN • G_2-4S • (2R_F - |Z_TR|)
Two-Wire to Four-Wire Gain
From (1) and (2) with V_RX = 0:
G_2-4
=
V_TX
V_TR
=
Z_T/α_RSN
Z_T
α_RSN • G_2-4S
- 2R_F
Four-Wire to Two-Wire Gain
From (1), (2) and (3) with E_L = 0:
G_4-2
=
V_TX
V_TR
=
Z_{T •}
Z_RX
Z_L
Z_T
α_RSN
-
G_2-4S
•
(
Z_L
+
2R_F)
In applications where
2R_F - α Z_T/( _RSN • G_2-4S) is chosen to be
equal to Z_L, the expression for G_4-2 simpli-
fies to:
G_4-2 = -
Z_T
Z_RX
•
1
2 • G_2-4S
Four-Wire to Four-Wire Gain
From (1), (2) and (3) with E_L = 0:
G_4-4
=
V_{TX =}
V_RX
ZZ_TRX•
G_2-4S • ( Z_L + 2R_F)
Z_T
α_RSN
-
G_2-4S
•
(
Z_L
+
2R_F)
10