PBL 386 11/2
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
V
RX
+
= 0 (E
L
= 0)
R
TX
Z
B
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:
V
Z
B
= - R
TX
•
RX
=
V
TX
Z
T
α
RSN
- G
2-4S
•
( Z
L
+ 2R
F
)
G
2-4S
•
( Z
L
+ 2R
F
)
The PBL 386 11/2 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.
Z
- R
TX
•
RX
•
Z
T
When choosing R
TX
, make sure the
output load of the VTX terminal is (R
TX
//R
T
in figure 13) > 20 kΩ.
If calculation of the Z
B
formula above
yields a balance network containing an
inductor, an alternate method is recom-
mended. Contact Ericsson Microelectron-
ics for assistance.
R
FB
VTX
R
TX
V
T
Z
T
Z
RX
Z
B
PBL
386 11/2
Combination
CODEC/Filter
V
RX
RSN
Figure 10. Hybrid function.
11
ERICSSON [ ERICSSON ]
