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PBL38650/2QNS

Subscriber Line Interface Circuit

Ericsson  

PBL 386 50/2

If calculation of the ZB formula above

yields a balance network containing an

inductor, an alternate method is recom-

mended. Contact Ericsson Microelectron-

ics for assistance.

The PBL 386 50/2 SLIC may also be

used together with programmable

CODEC/filters. The programmable

CODEC/filter allows for system controller

adjustment of hybrid balance to accom-

modate different line impedances without

change of hardware. In addition, the

transmit and receive gain may be

adjusted. Please, refer to the program-

mable CODEC/filter data sheets for

design information.

Longitudinal Impedance

A feed back loop counteracts longitudi-

nal 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 experi-

ence very small longitudinal voltage

excursions, leaving metallic voltages well

within the SLIC common mode range.

The SLIC longitudinal impedance per

wire, ZLoT and ZLoR, appears as typically

20Ω to longitudinal disturbances. It

should be noted that longitudinal currents

may exceed the dc loop current without

disturbing the vf transmission.

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, can be used for RFI filtering.

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 to a metallic imped-

ance of 1/(π·f·CTC) = 1/(π·f·CRC), a TIPX to

ground impedance of 1/(2·π·f·CTC) and a

RINGX to ground impedance of

1/(2·π·f·CRC).

VTX

RTX

PBL

386 50/2

ZT

ZB

Z RX

RSN

Figure 10. Hybrid function.

AC - DC Separation Capacitor, CHP

The high pass filter capacitor con-

nected between terminals HP and TIPX

provides the separation of the ac signal

from the dc part. CHP positions the low

end frequency response break point of

the ac loop in the SLIC. Refer to table 1

for recommended values of CHP.

Example: A CHP value of 150 nF will

position the low end frequency response

3dB break point of the ac loop at 1.8 Hz

(f3dB) according to f3dB = 1/(2·π·RHP·CHP)

where RHP = 600 kΩ.

High-Pass Transmit Filter

The capacitor CTX in figure 12 con-

nected between the VTX output and the

CODEC/filter forms, together with RTX

and/or the input impedance of a pro-

grammable CODEC/filter, a high-pass

RC filter. It is recommended to position

the 3 dB break point of this filter between

30 and 80 Hz to get a faster response for

the dc steps that may occur at DTMF

signalling.

Capacitor CLP

The capacitor CLP, which connects

between the terminals CLP and VBAT,

positions together with the resistive loop

feed resistor RSG (see section Battery

Feed), the high end frequency break

point of the low pass filter in the dc loop

in the SLIC. CLP together with RSG, CHP

and ZT (see section Two-Wire Imped-

RFB

VT

Combination

CODEC/Filter

VRX

ance) forms the total two wire output

impedance of the SLIC. The choise of

these programmable components have

an influence on the power supply

rejection ratio (PSRR) from VBAT to the

two wire side at sub-audio frequencies.

At these frequencies capacitor CLP also

influences the transversal to longitudinal

balance in the SLIC. Table 1 suggests

suitable values on CLP for different

feeding characteristics. Typical values of

the transversal to longitudinal balance

(T-L bal.) at 200Hz is given in table 1 for

the chosen values on CLP.

RFeed

RSG

CLP T-L bal. CHP

@200Hz

[Ω] [kΩ] [nF] [dB] [nF]

2·50 0

150 -46

47

2·200 60.4 100 -46

150

2·400 147 47 -43

150

2·800 301 22 -36

150

Table 1. RSG , CLP and CHP values for

different feeding characteristics.

11

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