PBL 386 10/2
Hybrid Function
RF1 and RF2 may be sufficient, but series
The PBL 386 10/2 SLIC may also be inductances can be added to form a sec-
usedtogetherwithprogrammableCODEC/ ond order filter. Current-compensated in-
filters. The programmable CODEC/filter ductors are suitable since they suppress
allows for system controller adjustment of common-modesignalswithminimuminflu-
hybrid balance to accommodate different enceonreturnloss.Recommendedvalues
line impedances without change of hard- for CTC and CRC are below 1 nF. Lower
ware. In addition, the transmit and receive values impose smaller degradation on re-
gain may be adjusted. Please, refer to the turn loss and longitudinal balance, but also
programmable CODEC/filter data sheets attenuate radio frequencies to a smaller
The hybrid function can easily be imple-
mented utilizing the uncommitted amplifier
inconventionalCODEC/filtercombinations.
Please, refer to figure 10. 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
for design information.
extent. The influence on the impedance
loop must also be taken into consideration
when programming the CODEC. CTC and
summing node. These currents can be Longitudinal Impedance
made to cancel by letting:
C
RC contribute to a metallic impedance of
Afeedbackloopcounteractslongitudinal
1/(π•f•CTC) = 1/(π•f•CRC), a TIPX to ground
impedance of 1/(2•π•f•CTC) and a RINGX to
ground impedance of 1/(2•π•f•CRC).
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
smalllongitudinalvoltageexcursions,leav-
VTX
RTX ZB
V
+
RX = 0 (EL = 0)
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:
AC - 3DC Separation Capacitor, CHP
The high pass filter capacitor connected
ing metallic voltages well within the SLIC between terminals HP and RINGX p r o -
common mode range. vides the separation of the ac and dc
TheSLIClongitudinalimpedanceperwire, signals. CHP positions the low end frequen-
VRX
VTX
ZB = - RTX
•
=
ZLoT and ZLoR, appears as typically 20 Ω to cyresponsebreakpointoftheacloopinthe
longitudinal disturbances. It should be not- SLIC. Refer to table 1 for recommended
ed that longitudinal currents may exceed value of CHP.
ZT
αRSN
- G2-4S • ( ZL + 2RF)
ZRX
•
ZT
the dc loop current without disturbing the vf
transmission.
Example: A CHP value of 68 nF will
position the low end frequency response
- RTX
•
G2-4S • ( ZL + 2RF)
3dBbreakpointoftheacloopat13 Hz(f3dB
according to f3dB = 1/(2•π•RHP•CHP) where
)
When choosing RTX, make sure the
output load of the VTX terminal is (RTX//RT
in figure 12) > 20 kΩ.
Capacitors CTC and CRC
RHP = 180 kΩ.
If RFI filtering is needed, the capacitors
designated CTC and CRC in figure 12, con-
nected between TIPX and ground as well
as between RINGX and ground, may be
mounted.
CTC and CRC work as RFI filters in con-
junction with suitable series impedances
(i.e. resistances, inductances). Resistors
If calculation of the ZB formula above
yields a balance network containing an
inductor, an alternate method is recom-
mended. Contact Ericsson Microelectron-
ics for assistance.
R
FB
R
TX
VTX
V
T
PBL
386 10/2
Z
Z
Combination
T
B
CODEC/Filter
Z
RX
V
RX
RSN
Figure 10. Hybrid function.
11
ERICSSON [ ERICSSON ]
