PBL 3766
Functional Description and Applications Information
Transmission
General
frequency which ensures stability and
reduces noise.
series with 2.16 µF (CL), RF = 40 ohms,
RTX = 20 kohms, G4-2 = -1. Calculate ZB.
Using the ZB formula above:
Two-wire to Four-wire gain
A simplified ac model of the transmission
circuits is shown in figure 8. Circuit
analysis yields:
ZRX 2ZL
The two-wire to four-wire gain, G2-4, is
obtained from (1) and (2) with VRX = 0:
ZB = {ZL = ZTR} = RTX·
·
=
ZT ZL + 2RF
VTX
VTR
ZT/1000
V
TR = VTX + IL · 2RF
VTX VRX IL
ZRX 1000
TR = EL - IL · ZL
(1)
ZL
=
G2-4
=
=
= {G4-2 = -1} = RTX·
ZT/1000 + 2RF
ZL + 2RF
+
=
(2)
Four-wire to Two-wire gain
ZT
1 + jω · RL · CL
1 + jω · (RL + 2RF) · CL
= RTX
·
The four-wire to two-wire gain, G4-2, is
derived from (1), (2) and (3) with EL = 0:
V
(3)
A network consisting of RB1 in series
with the parallel combination of RB and CB
has the same form as the required
where
VTR
VRX
ZT
ZL
G4-2
=
= -
·
ZRX ZT/1000 + 2RF + ZL
VTX is a ground referenced unity gain
version of the ac metallic voltage
between the TIPX and RINGX
balance network, ZB. Basic algebra yields:
Four-wire to Four-wire gain
RL
The four-wire to four-wire gain, G4-4, is
derived from (1), (2) and (3) with EL = 0:
terminals, i.e. VTX = 1 · VTRX
.
RB1 = RTX
RB = RTX
·
·
= 17.6 kohms
= 2353 ohms
= 0.62 µF
RL + 2RF
2RF
VTR is the ac metallic voltage between tip
and ring.
VTX
VRX
ZT
ZL + 2RF
G4-4
=
= -
·
ZRX ZT/1000 + 2RF + ZL
RL + 2RF
EL is the line open circuit ac metallic
voltage.
Hybrid Function
(RL + 2RF)2 · CL
RTX · 2RF
CB
=
The PBL 3766 SLIC forms a particularly
flexible and compact line interface when
used with programmable CODEC/filters.
The programmable CODEC/filters allows
for system controller adjustment of hybrid
balance to accommodate different line
impedances without change of hardware.
It also permits the system controller to
adjust transmit and receive gains as well
as terminating impedance. Refer to pro-
grammable CODEC/filter data sheets for
design information.
The hybrid function in an implementa-
tion utilizing the uncommitted amplifier in
a conventional CODEC/filter combination
is shown in figure 9. 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 is returned to VTX. This voltage is
converted by RTX to a current into the
same summing node. These currents
can be made to cancel by letting:
IL is the ac metallic current.
RF is a current limiting resistor in the
overvoltage protection network.
Longitudinal Impedance
In the active state, a feedback loop
counteracts longitudinal voltages at the
two-wire port by injecting longitudinal
currents in opposing phase. Therefore
longitudinal disturbances will appear as
longitudinal currents and the TIPX and
RINGX terminals will experience very
small longitudinal voltage excursions, well
within the SLIC common mode range.
This is accomplished by comparing the
instantaneous two-wire longitudinal
voltage to an internal reference voltage,
VBat/2. As shown below, the SLIC appears
as 20 ohms to ground per wire to longitu-
dinal disturbances. It should be noted,
that longitudinal currents may exceed the
dc loop current without disturbing the vf
transmission. From figure 10 the longitudi-
nal impedance can be calculated:
ZL is the line impedance.
ZT determines the SLIC TIPX to RINGX
impedance.
ZRX controls four- to two-wire gain.
VRX is the analog ground referenced
receive signal.
Two-wire Impedance
To calculate ZTR, the impedance
presented to the two-wire line by the SLIC
including the fuse resistors RF, let VRX = 0.
From (1) and (2):
ZT
ZTR
=
+ 2RF
1000
With ZTR and RF known ZT may be
calculated from
VLo
ILo
RLo
ZT = 1000 · (ZTR - 2RF)
=
= 20 ohms
1000
Example: calculate ZT to make the
terminating impedance ZTR = 600 ohms in
series with 2.16 µF. RF = 40 ohms.
Using the expression above
VTX VRX
where
+
= 0
(EL = 0)
RTX
ZB
VLo is the longitudinal voltage
ILo is the longitudinal current
Substituting the four-wire to four-wire
gain expression, G4-4, for VRX/VTX yields
the formula for a balanced network:
1
ZT = 1000 · (600 +
- 2 · 40)
RLo = 20 kohms sets the longitudinal
impedance
jω · 2.16 · 10-6
V
ZRX ZT/1000+2RF+ZL
i.e ZT = 520 kohms in series with 2.16 nF.
It is necessary to have a high ohmic
resistor in parallel with the capacitor. This
gives a DC-feedback loop, for low
ZB = -RTx· RX = RTX·
·
VTX
ZT
ZL + 2RF
Example: ZTR = ZL = 600 ohms (RL) in
4-10
ERICSSON [ ERICSSON ]
