[SI-LIST] Re: Gigabit ethernet placing and routing related question
- From: David A Mullenex <mullenex@xxxxxxxxx>
- To: Peter zhu <yonghui.sky@xxxxxxxxx>
- Date: Sat, 30 Oct 2010 10:39:39 -0700
Greetings all,
RJ45 connectors with integrated magnetics, LEDs, and CM-C are commonly used due
mostly to real-estate savings in high port count designs. Alas, a designer
cannot simply assume that the connector will take care of all emissions issues
in their designs.
Be cautioned that not all RJ45 with integrated magnetics are created equal.
Some suppliers have a better understanding of the electrical spectral content
and how to control such with consistent circuits embedded in the connectors.
Note also that the traditional front-end chassis / digital ground cuts need to
be reassessed with the use of integrated connectors.
Then the need for an external CM-C is really enclosure and design specific,
including the target copper port density for your product.
Regards,
- David.
On Oct 30, 2010, at 2:09 AM, "Peter zhu" <yonghui.sky@xxxxxxxxx> wrote:
> PHY output is analog signals, so keep them as short as possible.
> The trace between PHY and transformer refers to signal GND, and the trace
> between PHY and connector is chassis GND or Earth. So you have to cut the
> ground plane.
> Now there are so many RJ45 integrated transformer, this will keep transformer
> and connector very close. Why not use such RJ45?
> I have not seen separate CM choke, it is always the part of transformer.
> Another is keeping center tap power very clean, otherwise it will have
> Ethernet emission issue.
> Peter zhu
>
>
>
>
>
> --------------------------------------------------
> From: <jhasson@xxxxxxxxxxxxxxxxxxx>
> Sent: Thursday, October 28, 2010 4:57 AM
> To: <si-list@xxxxxxxxxxxxx>
> Subject: [SI-LIST] Gigabit ethernet placing and routing related question
>
>> Hi,
>> I am currently working on a project involving routing of gigabit ethernet
>> signals between a PHY and the outside world. Basically, after going
>> through the routing recommendations, it is most of the time recommended to
>> have the PHY close to the magnetics close to the RJ45 connector. The
>> closer the better.
>> I went though the si-list about this subject and I found some advice like
>> the transformer should be close to the phy but the CM choke should be
>> close to the connector. The idea of having an EMI protection device close
>> to the connector seems to make sense but I do not understand the reason
>> why the transformer should be close to the PHY : what is the advantage
>> versus having both the magnetics and the CM choke near the connector and
>> therefore a "long" distance between the PHY and the magnetics ?
>> Although having the CM choke near the connector seems to make sense I have
>> also read about the fact that it it was ok to have quite a distance
>> between the transformer/choke and the connector but no mention there about
>> some restrictions concerning the PHY to transformer distance.
>> Could someone, give (again ?) some insights into the pros and cons of each
>> topology ?
>>
>> Best regards,
>>
>> JF Hasson
>>
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