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From: eeh on 24 Apr 2005 02:13
Hi,

I saw the datasheet of pulse H1102 ethernet magnetic but cannot
understand the theory.

I want to know what is the function of the transformers interfacing to
the RJ45 ethernet network. Is it used to reject common mode noise?

Besides, why do I need to use the magnetic to interface to ethernet?

Thanks!

From: Andrew Holme on 24 Apr 2005 05:23
eeh wrote:
> Hi,
>
> I saw the datasheet of pulse H1102 ethernet magnetic but cannot
> understand the theory.
>
> I want to know what is the function of the transformers interfacing to
> the RJ45 ethernet network. Is it used to reject common mode noise?

Yes; and for isolation - to protect sensitive inputs from different mains
earth voltages around the building.

> Besides, why do I need to use the magnetic to interface to ethernet?

Well, if you're just experimenting - with short CAT5 cable runs - you don't.
This guy connected the pins of an FPGA directly to his hub for transmitting,
and just used a simple discrete-transistor differential-amplifier for
receiving:

http://www.fpga4fun.com/10BASE-T.html

I wouldn't do it without magnetics myself. It's not that hard. I used the
SI-10021 RJ45 connector with integral magnetics from RS Components in this
project:

http://www.holmea.demon.co.uk/Ethernet/EthernetRx.htm




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