[SI-LIST] Differential signals - skew - and EMC
- From: "Eric Bogatin" <eric@xxxxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Mon, 4 Nov 2013 09:32:47 -0700
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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