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PBL38661-2

Subscriber Line Interface Circuit

Ericsson  

Preliminary

PBL 386 61/2

Hybrid Function

The hybrid function can easily be imple-

mented utilizing the uncommitted amplifier

in conventional CODEC/filter combinations.

Please, refer to figure 10. Via impedance

ZB a current proportional to VRX is injected

into the summing node of the combination

CODEC/filter amplifier. As can be seen

from the expression for the four-wire to

four-wire gain a voltage proportional to VRX

is returned to VTX. This voltage is converted

by RTX to a current flowing into the same

summing node. These currents can be

made to cancel by letting:

VTX

RTX

+

VRX

ZB

=

0

(EL

=

0)

The four-wire to four-wire gain, G4-4, in-

cludes the required phase shift and thus

the balance network ZB can be calculated

from:

ZB

=

-

RTX

•

VRX

VTX

=

-

RTX

•

ZRX

ZT

•

ZT

αRSN

-

G2-4S

•

(

ZL

+

2RF)

G2-4S • ( ZL + 2RF)

When choosing RTX, make sure the

output load of the VTX terminal (RTX//RT

in figure 12) is > 20 kΩ.

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 61/2 SLIC may also be

used together with programmable CODEC/

filters. The programmable CODEC/filter

allows for system controller adjustment of

hybrid balance to accommodate different

line impedances without change of hard-

ware. In addition, the transmit and receive

gain may be adjusted. Please, refer to the

programmable CODEC/filter data sheets

for design information.

Longitudinal Impedance

A feed back loop counteracts longitudinal

voltages at the two-wire port by injecting

longitudinal currents in opposing phase.

Thus longitudinal disturbances will ap-

pear as longitudinal currents and the TIPX

and RINGX terminals will experience very

small longitudinal voltage excursions, leav-

ing metallic voltages well within the SLIC

common mode range.

The SLIC longitudinal impedance per wire,

ZLoT and ZLoR, appears as typically 20 Ω to

longitudinal disturbances. It should be not-

ed that longitudinal currents may exceed

the dc loop current without disturbing the vf

transmission.

Capacitors CTC and CRC

If RFI filtering is needed, the capacitors

designated CTC and CRC in figure 12, con-

nected between TIPX and ground as well

as between RINGX and ground, may be

mounted.

CTC and CRC work as RFI filters in con-

junction with suitable series impedances

(i.e. resistances, inductances). Resistors

RF1 and RF2 may be sufficient, but series

inductances can be added to form a sec-

ond order filter. Current-compensated in-

ductors are suitable since they suppress

common-mode signals with minimum influ-

ence on return loss. Recommended values

for CTC and CRC are below 1 nF. Lower

values impose smaller degradation on re-

turn loss and longitudinal balance, but also

attenuate radio frequencies to a smaller

extent. The influence on the impedance

loop must also be taken into consideration

when programming the CODEC. CTC and

CRC contribute to a metallic impedance 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).

AC - DC Separation Capacitor, CHP

The high pass filter capacitor connected

between terminals HP and RINGX p r o -

vides the separation of the ac and dc

signals. CHP positions the low end frequen-

cy response break point of the ac loop in the

SLIC. Refer to table 1 for recommended

value of CHP.

Example: A CHP value of 68 nF will

position the low end frequency response

3dB break point of the ac loop at 13 Hz (f3dB)

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

RHP = 180 kΩ.

RFB

VTX

RTX

PBL

386 61/2

ZT

ZB

RSN

Z RX

VT

Combination

CODEC/Filter

VRX

Figure 10. Hybrid function.

11

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