T604.F (05/12)

4

pulseelectronics.com

ISDN S-Interface Transformers

Surface Mount, Dual, 2000Vrms

1. General Information - The S-interface is the

standardized four-wire digital telephone access point

defined by the CCITTI I-series recommendations for

the Integrated Services Digital Network. This “basic

rate access” accommodates two 64Kpbs “B-channels”

for information, one 16Kbps “D-channel” intended

for signaling and control, and 48 Kbps for framing

and other purposes, giving a total rate of 192Kbps. The

CCITT physical layer recommends that the user network

interface be transformer coupled. A typical application

circuit is presented in Figure 1.

The transformer provides isolation for the line card

or the terminal from the line. It is also a way to provide

phantom power feeding to the terminal over the S-loop.

Each end requires a transmit and a receive transformer.

Chokes are used in some applications to reduce

common mode noise (see note 5).

The transformers described in this data sheet are

matched to the transceivers offered by the major IC

manufacturers listed. The use of a transformer-chip pair

assures that all requirements of CCITT I.430 are met

with respect to pulse waveform templates, impedance

and longitudinal balance. In addition, the transformers

provide the isolation voltages required by the regulatory

agencies and are capable of passing surge voltage tests.

2. Longitudinal Balance for Transformers - The

longitudinal conversion loss specification in I.430 includes

a test setup that is intended for system evaluation,

whether TE or NT. Such a test is performed with DC

current present. The transformer plays an essential role

in achieving the required balance. However, all other

elements in the circuit should be also designed to provide

the highest possible symmetry. These elements include

the protection circuitry, series resistors, chip transmitter

output and receiver input, as well as the cables used

and the TE cord where applicable. A high level of

transformer balance compensates for some circuit AC-

unbalance, and assures compliances of the system with

I.430 on longitudinal conversion loss (LCL) and signal

balance. This is accomplished by a well AC-balanced

winding configuration whereby the capacitive coupling

between primary and secondary windings is evenly

distributed. Also, both halves of the lineside winding are

well balanced. The transformers meet the following LCL

specifications:

•10kHzto300kHz:60dBminimum

•300kHzto1MHz:Minimumvaluedecreasing

20dB/decade.

(continued on next page)

IC

XMIT

Sec : Pri

RCV

High

Freq

Choke

IC

RCV

XMIT

Low

Freq

Choke

Low

Freq

Choke

High

Freq

Choke

S-INTERFACE

Local

Power

DC

Supply

Pri : Sec

NT

TE

Sec : Pri

Pri : Sec

Figure 1. Typical ISDN S-Interface Application Circuit