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Transformers and digital signals

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How can isolation transformers work digital signals such as Ethernet? Doesn't a transformer generate current on the secondary side when the magnetic flux in the core changes which only changes when the current on the primary side changes? If so, this would mean that the there would only be current spikes generated on the secondary side when the signal switches?

There is something I am missing obviously...

Ethernet uses Manchester encoding which doesn't require a DC component to work because the data is carried as transitions.

See: http://en.wikipedia.org/wiki/Manchester_code

Yes, I realize that, but between the switchings, di/dt is ideally zero which would give the spikes I described above. I'm still missing something I believe :)

It's all about frequency response. On the transformer, if you plot secondary voltage against primary voltage across a frequency range from DC to 100 MHz (say), then you will indeed see a signal level of zero at DC. But as the frequency rises, the response will increase until you get a close to 1:1 transfer (if the turns ration is 1:1). This response should remain relatively flat over the frequcny range the transformer is designed for. The Manchester encoding scheme guarantees a minimum frequncy component which means that the signal will pass through the transformer with little loss or distortion.

Ah, so the transfer improves as the frequency increases? Also, I interpret your comments about transformer design that this also depends on the way the transformer is designed, i.e. number of turns, core material, core shape et.c.

This makes sense to me since coming from an audio background (not engineering, music production), I've understood that better quality audio transformers primarily have better low end response.


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