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PBL38772/1

Subscriber Line Interface Circuit (SLIC)

Ericsson  

PBL 387 72

Z-filter in the codec). The SLIC SPICE model includes the

effects of CLP and CHP on the vf transmission. The CLP value

of 470 nF will position the high end frequency

response 3 dB break point of the ac loop at 0.3 Hz (f3dB).

Adaptive overhead voltage, AOV

The Adaptive Overhead Voltage feature minimizes the SLIC

power dissipation by permitting the TIPX and RINGX dc

voltages to operate very close to the supply rails. When the

SLIC detects a condition where the ac signal on TIPX/RINGX

is approaching the supply rail and therefore would become

distorted, the SLIC adjusts the overhead voltage, such that

the TIPX/RINGX dc bias is moved away from the rails

and thereby yielding enough peak signal swing for the ac

signal. High level signal conditions such as when voice and

metering signals are transmitted simultaneously are therefore

automatically accommodated for the duration of the high

level signal condition. This AOV system provides the designer

with a flexible solution for different system requirements and

possible future changes regarding voice, metering and other

signal levels. There is no dc overhead level that must be set

to a fixed value on account of worst case predicted peak

ac signal value. Overhead voltage is defined asthe voltage

between TIPX and RINGX or RINGX and VTB (depending on

selected state or used battery). The PBL387 72/1 will behave

as a SLIC with fixed overhead voltage for signals in the

0-20 kHz range and with an amplitude less than 1VPeak. For

signal amplitudes between 1VPeak and 1.25 VPeak the adaptive

overhead function will expand the overhead voltage making

it possible for the signal to propagate through the SLIC

without distortion. The expansion of the overhead occurs

instantaneously. When the signal amplitude decreases, the

overhead returns to its initial value with a time constant of

approximately one second.

During operation the influence of the adaptive overhead

function will not effect the SLIC performance in the constant

current region of operation. If, however, the SLIC is in the

off-hook, constant voltage region of operation, then the

influence of the adaptive headroom will be apparent as a

slight decrease in line voltage (and hence line current) as the

SLIC adjusts to accommodate the larger signal (e.g. voice +

metering).

Metering Applications

Subscriber Pulse Metering (SPM), also known as Advice-of-

Charge signaling (AOC), is used in several European countries

to provide the subscriber with an accurate indication of the

cost of a call in progress. Pulses of an out-of-speechband

signal are sent at the same time as the speech signal down

the telephone line, the rate of the pulses indicating the cost

of the call - faster pulse rates indicate a more expensive call.

An electronic meter at the subscriber counts the pulses as

they arrive and indicates the call cost on a digital display. This

meter is normally wired in parallel with the telephone circuit

and also provides filtering of the signal so that the subscriber

at the telephone does not hear it.

There are two frequencies used for SPM signaling: 12kHz

and 16kHz. The frequency used depends on the national

requirements. The frequency of the SPM signal must be

quite accurate, ±0.5% is typical. Furthermore the signal

must be sinusoidal with <5% total harmonic distortion. Pulse

metering signals can be applied to the two-wire line via the

PBL 387 72/1 SLIC by connecting the pulse-metering source

through coupling capacitor (CTTX) and resistor (RTTX) to the

RSN node. The capacitor in series isolates the RSN input

from any dc voltage that may be superimposed on the

metering signal. The signal level of metering has to be

included when optimizing talk battery VTB. It is possible to mix

speech and metering up to 1.7 VPeak using the AOV function.

The metering signal gain can be calculated from the equation:

α G4-2TTX =

VTRTTX

VRTTX

= - ZT × 1

ZTTX G2-4S

×

ZLTTX

ZT

RSN × G2-4S

+ZLTTX+2RF

where

VTRTTX is the desired metering voltage between the TIP and

RING terminals

VRTTX is the metering voltage injected via the resistor RTTX

ZLTTX is the line impedance seen by the 12 or 16 kHz

metering signal, typically 200 Ω

G2-4S is the transmit gain through the SLIC (0.5).

1 VPeak

2.50 V

2.50 V

2.50 V

Figure 11. The AOV function. (Observe that burst is

undersampled)

14

EN/LZT 146 136 R1A © Ericsson Microelectronics, December 2001

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