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isolating transformer for S/PDIF

029

75 Ω

Tr1

coax 75

6 : 6

TN13/7.5/5-3E25

994043 - 11

T. Giesberts

WARNING: do not use this transformer for mains isolating,

because its insulation is not capable of handling this. It is intended

to prevent earth loops arising or undesired signals being applied

to the input of an appliance. For instance, a tape recorder is to be

58

Elektor Electronics

7-8/99

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linked to a sound card in a computer which has an S/PDIF

(Sony/Philips Digital Interface Format); the computer is not con-

nected to protective earth (which in the UK is next to impossible

and certainly not advisable). Owing to the mains filter, half the

mains voltage will be present on the enclosure and thus on the

input earth. The linking of this potential to the tape recorder is

prevented by the isolating transformer in the diagram.

To ensure a good bandwidth, the coupling factor of the trans-

former must be good (low stray self-inductance), so that a core

with a high µ

elled copper wire, are laid on opposite sides of the toroid. The

windings should be covered with insulating tape. If heavy-duty,

insulated wire is used for the windings, they can be laid over one

another, which improves the coupling factor. But even with the

first method, the bandwidth ranged from 50 kHz to 17 MHz, which

is more than adequate for an S/PDIF link.

Place the transformer directly at the output of the signal source.

The reason for this is that the input and output impedances of the

transformer are not exactly 75 Ω . With the transformer directly at

the source and provided the coaxial cable at the computer end is ter-

minated correctly into 75 Ω

, all will be well.

[994043]

is needed. The prototype uses a Philips Type

TN13/7.5/5-3E25, which has a µ τ of 4500. The primary and sec-

ondary windings, each consisting of six turns of 0.5 mm dia. enam-

τ

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