PBL38665-2 Datasheet, PDF (11/18 Pages) Ericsson – Subscriber Line Interface Circuit

Preliminary
PBL 386 65/2
Hybrid Function
The hybrid function can easily be imple-
mented utilizing the uncommitted amplifier
in conventional CODEC/filter combinations.
Please, refer to figure 10. Via impedance
Z_B a current proportional to V_RX 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 V_RX
is returned to V_TX. This voltage is converted
by R_TX to a current flowing into the same
summing node. These currents can be
made to cancel by letting:
V_TX
R_TX
+
V_RX
Z_B
=
0
(E_L
=
0)
The four-wire to four-wire gain, G_4-4, in-
cludes the required phase shift and thus
the balance network Z_B can be calculated
from:
Z_B
=
-
R_TX
•
V_RX
V_TX
=
-
R_TX
•
Z_RX
Z_T
•
Z_T
α_RSN
-
G_2-4S
•
(
Z_L
+
2R_F)
G_2-4S • ( Z_L + 2R_F)
When choosing R_TX, make sure the
output load of the VTX terminal is
> 20 kΩ.
If calculation of the Z_B formula above
yields a balance network containing an
inductor, an alternate method is recom-
mended.
The PBL 386 65/2 SLIC may also be
used together with programmable CODEC/
filters. The programmable CODEC/filter
allows for system controller adjustment of
hybrid balance to accommodate different
line impedances without change of hard-
ware. In addition, the transmit and receive
gain may be adjusted. Please, refer to the
programmable CODEC/filter data sheets
for design information.
Longitudinal Impedance
A feed back loop counteracts longitudinal
voltages at the two-wire port by injecting
longitudinal currents in opposing phase.
Thus longitudinal disturbances will ap-
pear as longitudinal currents and the TIPX
and RINGX terminals will experience very
small longitudinal voltage excursions, leav-
ing metallic voltages well within the SLIC
common mode range.
The SLIC longitudinal impedance per wire,
Z_LoT and Z_LoR, appears as typically 20 Ω to
longitudinal disturbances. It should be not-
ed that longitudinal currents may exceed
the dc loop current without disturbing the vf
transmission.
Capacitors C_TC and C_RC
If RFI filtering is needed, the capacitors
designated C_TC and C_RC in figure 13, con-
nected between TIPX and ground as well
as between RINGX and ground, may be
mounted.
C_TC and C_RC work as RFI filters in con-
junction with suitable series impedances
(i.e. resistances, inductances). Resistors
R_F1 and R_F2 may be sufficient, but series
inductances can be added to form a sec-
ond order filter. Current-compensated in-
ductors are suitable since they suppress
common-mode signals with minimum influ-
ence on return loss. Recommended values
for C_TC and C_RC are below 1 nF. Lower
values impose smaller degradation on re-
turn loss and longitudinal balance, but also
attenuate radio frequencies to a smaller
extent. The influence on the impedance
loop must also be taken into consideration
when programming the CODEC. C_TC and
C_RC contribute to a metallic impedance of
1/(π•f•C_TC) = 1/(π•f•C_RC), a TIPX to ground
impedance of 1/(2•π•f•C_TC) and a RINGX to
ground impedance of 1/(2•π•f•C_RC).
AC - DC Separation Capacitor, C_HP
The high pass filter capacitor connected
between terminals HP and RINGX p r o -
vides the separation of the ac and dc
signals. C_HP positions the low end frequen-
cy response break point of the ac loop in the
SLIC. Refer to table 1 for recommended
value of C_HP.
Example: A C_HP value of 68 nF will
position the low end frequency response
3dB break point of the ac loop at 13 Hz (f_3dB)
according to f_3dB = 1/(2•π•R_HP•C_HP) where
R_HP = 180 kΩ.
R_FB
VTX
R_TX
PBL
386 65/2
Z_T
Z_B
Z _RX
RSN
Figure 10. Hybrid function.
V_T
Combination
CODEC/Filter
V_RX
11