PBL 386 14/1
Hybrid Function
The PBL 386 14/1 SLIC may also be (i.e. resistances, inductances). Resistors
usedtogetherwithprogrammableCODEC/ RF1 and RF2 may be sufficient, but series
filters. The programmable CODEC/filter inductances can be added to form a sec-
allows for system controller adjustment of ond order filter. Current-compensated in-
hybrid balance to accommodate different ductors are suitable since they suppress
line impedances without change of hard- common-modesignalswithminimuminflu-
ware. In addition, the transmit and receive enceonreturnloss.Recommendedvalues
gain may be adjusted. Please, refer to the for CTC and CRC are below 1 nF. Lower
programmable CODEC/filter data sheets values impose smaller degradation on re-
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
summing node. These currents can be
made to cancel by letting:
for design information.
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. CTC and
CRC contribute to a metallic impedance of
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).
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
smalllongitudinalvoltageexcursions,leav-
ing metallic voltages well within the SLIC
common mode range.
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 - DC Separation Capacitor, CHP
The high pass filter capacitor connected
TheSLIClongitudinalimpedanceperwire, between terminals HP and RINGX p r o -
VRX
VTX
ZLoT and ZLoR, appears as typically 20 Ω to vides the separation of the ac and dc
ZB = - RTX
•
=
longitudinal disturbances. It should be not- signals. CHP positions the low end frequen-
ed that longitudinal currents may exceed cyresponsebreakpointoftheacloopinthe
the dc loop current without disturbing the vf SLIC. Refer to table 1 for recommended
ZT
- G2-4S • ( ZL + 2RF)
αRSN
ZRX
•
ZT
transmission.
value of CHP.
- RTX
•
G2-4S • ( ZL + 2RF)
Example: A CHP value of 68 nF will
position the low end frequency response
Capacitors CTC and CRC
When choosing RTX, make sure the
output load of the VTX terminal is (RTX//RT
in Figure 14) > 20 kΩ.
3dBbreakpointoftheacloopat13 Hz(f3dB
)
If RFI filtering is needed, the capacitors
designated CTC and CRC in figure 13, con-
nected between TIPX and ground as well
as between RINGX and ground, may be
mounted.
according to f3dB = 1/(2•π•RHP•CHP) where
RHP = 180 kΩ.
If calculation of the ZB formula above
yields a balance network containing an
inductor, an alternate method is recom-
mended.
CTC and CRC work as RFI filters in con-
junction with suitable series impedances
R
TX
VTX
V
T
PBL
386 14/1
Z
Z
Combination
CODEC/Filter
T
B
Z
RX
V
RX
RSN
Figure 10. Hybrid function.
11
ERICSSON [ ERICSSON ]
