PBL 386 61/2
Preliminary
Hybrid Function
The PBL 386 61/2 SLIC may also be RF1 and RF2 may be sufficient, but series
usedtogetherwithprogrammableCODEC/ inductances can be added to form a sec-
filters. The programmable CODEC/filter ond order filter. Current-compensated in-
allows for system controller adjustment of ductors are suitable since they suppress
hybrid balance to accommodate different common-modesignalswithminimuminflu-
line impedances without change of hard- enceonreturnloss.Recommendedvalues
ware. In addition, the transmit and receive for CTC and CRC are below 1 nF. Lower
gain may be adjusted. Please, refer to the values impose smaller degradation on re-
programmable CODEC/filter data sheets turn loss and longitudinal balance, but also
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
for design information.
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
by RTX to a current flowing into the same Longitudinal Impedance
summing node. These currents can be
made to cancel by letting:
A feed back loop counteracts longitudinal
CRC contribute to a metallic impedance of
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-
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).
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
ing metallic voltages well within the SLIC The high pass filter capacitor connected
common mode range. between terminals HP and RINGX p r o -
TheSLIClongitudinalimpedanceperwire, vides the separation of the ac and dc
ZLoT and ZLoR, appears as typically 20 Ω to signals. CHP positions the low end frequen-
longitudinal disturbances. It should be not- cyresponsebreakpointoftheacloopinthe
ed that longitudinal currents may exceed SLIC. Refer to table 1 for recommended
the dc loop current without disturbing the vf value of CHP.
VRX
VTX
ZB = - RTX
•
=
ZT
- G2-4S • ( ZL + 2RF)
αRSN
ZRX
•
ZT
transmission.
Example: A CHP value of 68 nF will
- RTX
•
G2-4S • ( ZL + 2RF)
position the low end frequency response
3dBbreakpointoftheacloopat13 Hz(f3dB
according to f3dB = 1/(2•π•RHP•CHP) where
HP = 180 kΩ.
)
Capacitors CTC and CRC
When choosing RTX, make sure the
output load of the VTX terminal (RTX//RT
in figure 12) is > 20 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
R
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 61/2
Z
Z
Combination
T
B
CODEC/Filter
Z
RX
V
RX
RSN
Figure 10. Hybrid function.
11
ERICSSON [ ERICSSON ]
