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PBL38650-2

Subscriber Line Interface Circuit

Ericsson  

PBL 386 50/2

+

TIP

ZL

VTR

ZTR

+

EL

-

-

RING

TIPX

RF RP

RF RP

RINGX

+

RHP

G 2-4S

-

IL

VTX

IL

ZT

+

VTX

-

PBL 386 50/2

RSN

I L /αRSN

ZRX

+

VRX

-

Figure 9. Simplified ac transmission circuit.

Functional Description

and Applications

Information

Transmission

General

A simplified ac model of the transmis-

sion circuits is shown in figure 9. Circuit

analysis yields:

VTR

= VTX

G2 − 4S

+ IL

⋅ (2RF

+ 2RP )

(1)

VTX + VRX = IL

(2)

ZT ZRX αRSN

VTR = EL - IL · ZL

(3)

where:

VTX is a ground referenced version of

the ac metallic voltage between the

TIPX and RINGX terminals.

G2-4S is the programmable SLIC two-wire

to four-wire gain (transmit

direction). See note below.

VTR is the ac metallic voltage between

tip and ring.

EL is the line open circuit ac metallic

voltage.

IL is the ac metallic current.

RF is a fuse resistor.

RP is part of the SLIC protection

ZL is the line impedance.

ZT determines the SLIC TIPX to

RINGX impedance at voice

frequencies.

ZRX controls four- to two-wire gain.

VRX is the analog ground referenced

receive signal.

αRSN is the receive summing node current

to metallic loop current gain = 200.

Note that the SLICs two-wire to four-

wire gain, G2-4S, is user programmable

between two fix values. Refer to the

datasheets for values on G2-4S.

Two-Wire Impedance

To calculate ZTR, the impedance

presented to the two-wire line by the

SLIC including the fuse and protection

resistors RF and RP, let VRX = 0.

From (1) and (2):

ZTR

=

ZT

αRSN ⋅ G 2− 4S

+

2RF

+

2RP

Thus with ZTR, αRSN, G2-4S, RP and RF

known:

ZT = αRSN ⋅ G2− 4S ⋅ (Z TR − 2RF − 2RP )

Two-Wire to Four-Wire Gain

From (1) and (2) with VRX = 0:

G2− 4

=

VTX

VTR

=

ZT / αRSN

ZT

αRSN ⋅ G2 − 4S

+

2RF

+

2RP

Four-Wire to Two-Wire Gain

From (1), (2) and (3) with EL = 0:

G4 −2

=

VTR

VRX

=

− ZT

ZRX

⋅

ZT

αRSN

ZL

+ G2 − 4S ⋅ ( ZL

+ 2RF

+ 2RP)

For applications where

ZT/(αRSN·G2-4S) + 2RF + 2RP is chosen to

be equal to ZL the expression for G4-2

simplifies to:

G4 −2

=−

ZT

ZRX

⋅

1

2G2 − 4S

Four-Wire to Four-Wire Gain

From (1), (2) and (3) with EL = 0:

G4 −4

=

VTX

VRX

=

−

ZT

ZRX

⋅

G 2− 4S ⋅ ( ZL + 2RF + 2RP)

ZT

αRSN

+ G2 −4 S ⋅ ( ZL

+ 2RF

+

2RP )

Hybrid Function

The hybrid function can easily be

implemented 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 ampli-

fier. 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,

includes the required phase shift and

thus the balance network ZB can be

calculated from:

ZB

=

−RTX

⋅

VRX

VTX

=

R TX

⋅ ZRX

ZT

⋅

ZT

α RSN

+ G2 −4 S ⋅ (ZL

+ 2RF

+ 2RP )

G2 −4 S ⋅ (ZL + 2RF + 2RP )

When choosing RTX, make sure the

output load of the VTX terminal is >20kΩ.

10

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