PBL 3764A/4, /6
small longitudinal voltage excursions,
leaving metallic voltages well within the
SLIC common mode range. This is
accomplished by comparing the
instantaneous two-wire longitudinal
voltage to an internal longitudinal
RLoT = RLoR = longitudinal resistance/wire
VLo = longitudinal voltage at TIPX, RINGX ring dc conditions and circuits processing
ILo = longitudinal current.
separation between circuits sensing tip-
ac signals. A CHP value of 10 nF will
position the low end frequency response
3dB break point at 48 Hz (f3dB) according
Capacitors CTC and CRC
The capacitors designated CTC and CRC in
figure 12, connected between TIPX and
ground as well as between RINGX and
ground, are recommended as an addition
to the overvoltage protection network. Very
fast transients, appearing on tip and ring,
may pass by the diode and SCR clamps in
the overvoltage protection network, before
these devices have had time to activate
and could damage the SLIC. CTC and CRC
short such very fast transients to ground.
The recommended value for CTC and CRC is
2200 pF. Higher capacitance values may
be used, but care must be taken to prevent
degradation of either longitudinal balance
or return loss. CTC and CRC contribute a
metallic impedance of 1/(π · f · CTC) ≈
1/( π · f · CRC), a TIPX to ground
to f3dB = 1/(2 • π • RHP • CHP) where RHP
330 kΩ.
≈
reference voltage, VLoRef
.
Battery Feed
VBat
VT
+ VR
VLoRef
=
=
The block diagram in figure 13 shows the
PBL 3764A/4 battery feed system.
For a tip to ring dc voltage VTR less than
the saturation guard reference voltage
VSGRef, the SLIC emulates a constant-
current feed characteristic. The constant
current is independent of the actual
battery voltage, VBat, connected to the
SLIC.
2
2
where VT and VR are tip and ring ground
referenced voltages without any
longitudinal component. As shown below,
the SLIC appears as 20 Ω per wire to
longitudinal disturbances. It should be
noted that longitudinal currents may
exceed the dc loop current without
disturbing the vf transmission. Refer to
figure 11.
With the tip to ring DC voltage VTR
exceeding VSGRef, the feed characteristic
changes to a nearly-constant voltage
feed. This is to prevent the tip and ring
drive amplifiers from distorting the AC
signal as might have otherwise occurred
due to insufficient voltage margin between
VTR and VBat (pin 6). Thus the SLIC
automatically adjusts the tip to ring dc
voltage VTR to the maximum safe value.
With the SLIC in the stand-by state (C1,
C2 = 1,1) a resistive feed characteristic is
enabled.
Circuit analysis yields:
VLo
ILo
=
impedance of 1/(2 · π · f · CTC) and a
RINGX to ground impedance of
1/(2 · π · f · CRC).
R
1000
Lo
which reduces to
RLoT = R LoR = VLo/ILo = 20 kΩ/1000 = 20 Ω
where:
RLo = 20 kΩ
Ac - dc Separation Capacitor, CHP
The high pass filter capacitor connected
between terminals 21 and 20 provides the
R
FB
The following text explains the three
battery feed cases in more detail.
Case 1: SLIC in the Active State;
R
TX
VTX
21
-
+
VTR < VSGRef
.
V
T
PBL
3764A/4
In the active state C1 = 0 and C2 = 1. In
this operating state tip to ring voltages VTR
less than VSGRef cause the block titled
saturation guard (figure 13) to be
Z
Z
Combination
CODEC/Filter
T
B
Z
RX
V
RX
19
RSN
disabled, i.e. its output is equal to zero.
For this case circuit analysis yields:
Figure 10. Hybrid function.
2.5V
RDC1 + RDC2
=
•1000
ILdc
V
+V
where:
ILdc
T
Lo
TIPX
5
TIP
I
Lo
R
20
= constant loop current
(independant of the loop
resistance RL)
F
V
+V
2
T
R
R
/2
/2
HP
+V
Lo
1
/
2
2
HPT 22
+
V
+
LO
C
HP
1
HPR
1
6
-
RDC1 + RDC2 = the programming
resistance which sets the
1
/
V
R
20
R
LoRef
F
R
HP
Lo
20K
constant loop current
For tip to ring voltages VTR less than
VSGRef the PBL 3764A/4 thus emulates a
constant current feed with the magnitude
of the constant current set by the
RING
I
Lo
RINGX
V
+V
Lo
R
I
/1000
Lo
resistors, RDC1 and RDC2
.
PBL 3764A/4
Capacitor CDC at the RDC1 - RDC2
common point removes vf signals from
the battery feed control loop. CDC is
calculated according to:
Figure 11. Longitudinal impedance.
11
ERICSSON [ ERICSSON ]
