PBL 3764A/4, /6
Functional Description and Applications Information
Four-Wire to tTwo-Wire Gain
proportional to VRX is returned to VTX. This
voltage is converted by RTX to a current
flowing into the same summing node.
These currents can be made to cancel by
letting:
Transmission
General
From (1), (2) and (3) with EL = 0:
V
ZT
ZL
A simplified ac model of the transmission
circuits is shown in figure 9. Circuit
analysis yields:
G4-2
=
TR = -
VRX
•
ZRX ZT/1000 + 2RF + ZL
For applications where ZT/1000 + 2RF is
chosen to be equal to ZL, the expression
for G4-2 simplifies to:
VTX VRX
RTX ZB
=
= 0 (EL = 0)
+
VTR = VTX + IL• 2RF
(1)
(2)
(3)
The four-wire to four-wire gain, G4-4,
includes the required phase shift and
thus the balance network ZB can be
calculated from:
VTX
ZT
VRX
ZRX
IL
1000
ZT
ZRX
1
2
+
=
G4-2 = -
•
VTR = EL – IL • ZL
Four-Wire to Four-Wire Gain
where:
VRX
VTX
From (1), (2) and (3) with EL = 0:
ZB = - RTX •
=
VTX is a ground referenced unity gain
version of the ac metallic voltage
between the TIPX and RINGX
terminals.
VTR is the ac metallic voltage between
tip and ring.
V
ZT
ZL + 2RF
G4-4
=
TX = –
VRX
•
ZRX
ZT
ZT/1000 + 2RF + ZL
ZL + 2RF
ZRX ZT/1000 + 2RF + ZL
= RTX
•
•
Hybrid Function
Example:
The PBL 3764A/4 SLIC forms a
Calculate RB for the line interface shown
in figure 12.
particularly flexible and compact line
interface when used with programmable
CODEC/filters. The programmable
CODEC/filter allows for system controller
adjustment of hybrid balance to
accommodate different line impedances
without change of hardware. In addition,
the transmit and receive gain may be
adjusted. Please, refer to the
EL
is the line open circuit ac metallic
voltage.
261 • 103 523 • 103 / 1000 + 2 • 40 +
=
600
RB = 20 • 103 •
523 • 103
•
IL
is the ac metallic current.
is a fuse resistor.
is the line impedance.
determines the SLIC TIPX to
RINGX impedance.
600 + 2 • 40
RF
ZL
ZT
= 17.66 kΩ (i.e. standard value
17.8 kΩ, 1%)
If calculation of the ZB formula above
yields a balance network containing an
inductor, an alternate method is
recommended. Contact Ericsson
Microelectronics for assistance.
ZRX controls four- to two-wire gain.
VRX is the analog ground referenced
receive signal.
programmable CODEC/filter data sheets
for design information.
The hybrid function can also be
Two-Wire Impedance
implemented utilizing the uncommitted
amplifier in conventional CODEC/filter
combinations. 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
Longitudinal Impedance
To calculate ZTR, the impedance
presented to the two-wire line by the
SLIC including the fuse resistors RF, let:
A feed back loop counteracts longitudinal
voltages at the two-wire port by injecting
longitudinal currents in opposing phase.
Thus longitudinal disturbances will appear
as longitudinal currents and the TIPX and
RINGX terminals will experience very
VRX = 0.
From (1) and (2): ZTR = ZT/1000 + 2RF
Thus with ZTR and RF known:
ZT = 1000•(ZTR – 2RF)
Example:
Calculate ZT to make ZTR = 900Ω in
series with 2.16 µF. RF = 40 Ω
TIPX
5
TIP
I
+
L
R
F
1
Z
L
ZT = 1000 • (900 +
– 2 • 40)
Z
+
TR
jω•2.16 • 10-6
TR
21 VTX
V
RHP
+
1
-
+
which yields:
E
L
V
TX
R
-
F
-
ZT = 820 kΩ in series with 2.16 nF.
I
L
RING
RINGX
6
-
Z
T
It is always necessary to have a high
ohmic resistor in parallel with the
capacitor. This gives a DC-feedback loop
for low frequency which ensures stability
and reduces noise.
Z
RX
19 RSN
/1000
+
V
RX
I
L
-
Two-Wire to Four-Wire Gain
PBL 3764A/4
From (1) and (2) with VRX = 0:
VTX
ZT/1000
Figure 9. Simplified ac transmission circuit.
G2-4
10
=
=
VTR ZT/1000 + 2RF
ERICSSON [ ERICSSON ]
