HC55151CM datasheet(23/35 Pages) INTERSIL

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4-23

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 2k

Ω 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 and the 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 (ZTR), at the pulse metering frequencies,

to be approximately 200

Ω. If the line impedance (ZL) is equal

to 200

Ω at the pulse metering frequencies, then the 4-Wire

to 2-wire gain (VTR / 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 VRX/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

(ZTR), including the protection resistors, equal to 200Ω. The

results are shown in Equation 40.

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.

Overload Levels and Silent Polarity Reversal

The pulse metering signal and voice are simultaneously

transmitted, and therefore require additional overhead to

prevent distortion of the signal. Reference section “Off hook

Overhead Voltage” to account for the additional pulse

metering signal requirements.

Most of the SLICs in the UniSLIC14 family feature full

polarity reversal. Full polarity reversal means that the SLIC

can: transmit, determine the status of the line (on hook and

off hook) and provide “silent” polarity reversal. Reference

Equation 39 to program the polarity reversal time.

FIGURE 22. 4-WIRE AND 2-WIRE LOOPBACK TESTS

VTX

VRX

TIP

RING

4-WIRE LOOPBACK

UniSLIC14

600

Ω

DUAL SUPPLY

CODEC/FILTER

INTERNAL

2-WIRE LOOPBACK

PTG

4-2 =

SPM

-------------

–

L +ZTR

-------------------------

200

200 + 200

---------------------------

–

(EQ. 40)

FIGURE 23. PULSE METERING WITH TRANSHYBRID

BALANCE

VTX

VRX = 0

500K

PTG

A = 1

500K

UniSLIC14

1/80K

125K

SPM

12/16kHz

PULSE METERING

INPUT SIGNAL

SETS 2-WIRE

IMPEDANCE

AT 12-16kHz

EQUAL TO

200

Ω

HC55120, HC55121, HC55130, HC55131, HC55140, HC55141, HC55142, HC55143, HC55150, HC55151

HC55151CM Datasheet(PDF) 23 Page - Intersil Corporation

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