[SI-LIST] Re: Differential signals - skew - and EMC
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 04 Nov 2013 10:45:49 -0800
Eric, those are all good points. The only exception that I take is to
statements about signals with tight return paths not radiating. They do
radiate. They don't radiate excessively. Doing foolish things like
passing signals, including differential signals, over slots increases
radiated power.
Olive oil entrepreneur and sometimes EMC engineer Don Corleone to his
son: "Keep your signal paths close. Keep your return paths closer."
Best Regards,
Steve
On 11/4/2013 8:32 AM, Eric Bogatin wrote:
> Hi folks-
>
>
> Here is an EDN blog I wrote a few weeks ago related to common currents and
> Charles Grasso's question about how much is too much.
>
>
>
> http://www.edn.com/electronics-blogs/test-voices/4420453/Not-all-common-curr
> ents-are-bad
>
>
>
>
>
> In my Advanced GigaBit Channel Design class, we go through the numbers. Here
> is the brief analysis:
>
>
>
> It takes about 3 uA of common current on Unshielded Twisted Pair (UTP) cable
> to fail FCC part 15 class B at about 100 MHz on a 1 m long external cable.
>
>
>
> It takes about 1 mV of common signal driving the cable to drive 3 uA and
> fail the test.
>
>
>
> If the diff signal is 600 mV (like PCIe II), this is -55 dB of mode
> conversion from the source to the UTP front end to fail a radiated emissions
> test.
>
>
>
> How can any product ship with an RJ45 connector for UTW cables if this is
> the limit to mode conversion, at the ~ 100 MHz point?
>
>
>
> The solution is common mode chokes. With a 40 dB isolation, this allows as
> much as -15 dB mode conversion before the common currents that get through
> cause EMI problems.
>
>
>
> However, even with shielded twisted pair, a bad connector negates the value
> of the shield proving the return for the common signal.
>
>
>
> How much radiated emissions you get is a function of how 360 degrees the
> connector is, hard to predict.
>
>
>
> This is why, as a "habit", do everything that is free to minimize
> asymmetries. When it cost more, use analysis to calculate the bang for the
> buck.
>
>
>
> This is just one example of how common currents on external cables can cause
> radiated emissions.
>
>
>
> --eric
>
>
>
>
>
>
>
> *******************************************************
> Dr. Eric Bogatin, Signal Integrity Evangelist
>
> Bogatin Enterprises
>
> Setting the Standard for Signal Integrity Training
> web site: <http://www.bethesignal.com/> www.beTheSignal.com
>
> Blog: <http://www.bethesignal.com/blog> www.beTheSignal.com/blog
>
> Twitter @beTheSignal
> e: <mailto:eric@xxxxxxxxxxxxxxx> eric@xxxxxxxxxxxxxxx
>
> Laboratory: Connected Community Networks
>
> 105 S Sunset St, Suite J
>
> Longmont, CO 80501 USA
>
> cell: 913-424-4333 skype: eric.bogatin
> ***********************************************
>
>
>
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of Grasso, Charles
>
> Sent: Friday, November 01, 2013 12:32 PM
>
> To: si-list@xxxxxxxxxxxxx
>
> Subject: [SI-LIST] Differential signals - skew - and EMC
>
>
>
> Hello,
>
>
>
> In an ideal situation differential signals will have no skew through the
> transmission path and (as I understand it) the common-signal (emi) will be
> very low as a result. Given that EMC is very system dependent - does any
> one have a rule of thumb or anxiety(!) factor for how much is skew is
> tolerable before becoming an emissions (regulatory) Issue?
>
>
>
> Thanks in advance.
>
> Charles Grasso
>
>
>
>
>
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--
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