HC55120_06 Datasheet, PDF (24/36 Pages) Intersil Corporation – Low Power Universal SLIC Family

HC55120, HC55121, HC55130, HC55140, HC55142, HC55143, HC55150
500K
I_X
500K
-
+
A=1
V_TX
PTG
R1
-
V_TX
+
R2
I_X
500K
V_RX
500K
+
V_RX
5
-
UniSLIC14
FIGURE 21. TRANSHYBRID BALANCE USING THE PTG PIN
Loopback Tests
4-Wire Loopback Test
This feature can be very useful in the testing of line cards
during the manufacturing process and in field use. The test
is unobtrusive, allowing it to be used in live systems.
Reference Figure 22.
Most systems do not provide 4-wire loopback test capability
because of costly relays needed to switch in external loads.
All the SLICs in the UniSLIC14 family can easily provide
this function when configured in the Open Circuit logic
state. With the PTG pin floating, the signal on the V_TX
output is 180° out of phase and approximately 2 times the
V_RX input signal. If the PTG pin is grounded, then the
amplitude will be approximately the same as the input
signal and 180° out of phase.
TIP
RING
UniSLIC14
INTERNAL
600Ω
V_TX
PTG
V_RX
-
+
DUAL SUPPLY
CODEC/FILTER
2-WIRE LOOPBACK
4-WIRE LOOPBACK
FIGURE 22. 4-WIRE AND 2-WIRE LOOPBACK TESTS
2-Wire Loopback Test
Most of the SLICs in the UniSLIC14 family feature 2-Wire
loopback testing. This loopback function is only activated
when the subscriber is on hook and the logic command to
the SLIC is in the Test Active State. (Note: if the subscriber is
off hook and in the Test Active State, the function performed
is the Line Voltage Measurement.)
During the 2-wire loopback test, a 2kΩ internal resistor is
switched across the tip and ring terminals of the SLIC. This
allows the SHD function and the 4-wire to 4-wire AC
transmission, right up to the subscriber loop, to be tested.
Together with the 4-wire loopback test in the Open Circuit
logic state, this 2-wire loopback test allows the complete
network (including SLIC) to be tested up to the subscriber
loop.
Pulse Metering
The HC55121, HC55142, HC35143, and HC55150 are
designed to support pulse metering. They offer solutions to
the following pulse metering design issues:
1) Providing adequate signal gain and current drive to the
subscriber metering equipment to overcome the attenuation
of this (12kHz, 16kHz) out of band signal.
2) Attenuating the pulse metering transhybrid signal without
severely attenuating the voice band signal to avoid clipping
in the CODEC/Filter.
3) Tailoring the overload levels in the SLIC to avoid clipping
of the combined voiceband and pulse metering signal.
4) Having the provision of silent polarity reversal as a backup
in the case where the loop attenuates the out of band signal
too much for it to be detected by the subscriber’s metering
equipment.
Adequate Signal Gain
Adequate signal gain and current drive to the subscriber’s
metering equipment is made easier by the network shown in
Figure 23. The pulse metering signal is supplied to a
dedicated high impedance input pin called SPM. The circuit
in Figure 23 shows the connection of a network that sets the
2-wire impedance (Z_TR), at the pulse metering frequencies,
to be approximately 200Ω. If the line impedance (Z_L) is equal
to 200Ω at the pulse metering frequencies, then the 4-Wire
to 2-wire gain (V_TR/SPM) is equal 4. Thereby lowering the
input signal requirements of the pulse metering signal.
Note: The automatic pulse metering 2-wire impedance
matching is independent of the programmed 2-wire
impedance matching at voiceband frequencies.
Calculation of the pulse metering gain is achieved by
replacing V_RX/500k in Equation 15 with SPM/125k and
following the same process through to Equation 21. The
UniSLIC14 sets the 2-wire input impedance of the SLIC
(Z_TR), including the protection resistors, equal to 200Ω. The
results are shown in Equation 41.
A_4-2
=
--^V----^T---^R----
SPM
=
–8
----------^Z----^L-----------
Z_L + Z_TR
=
–8
---------²----⁰---⁰-----------
200 + 200
=
–4
(EQ. 41)
Avoiding Clipping in the CODEC/Filter
The amplitude of the returning pulse metering signal is often
very large and could easily over drive the input to the
CODEC/Filter. By using the same method discussed in
section “Transhybrid Balance”, most if not all of the pulse
metering signal can be canceled out before it reaches the
input to the CODEC/Filter. This connection is shown in
Figure 23.
24
FN4659.13
June 1, 2006