[SI-LIST] Re: Emission level vary for same type of SATA cables but different vendors
- From: "McCoy, Bart" <McCoy.Bart@xxxxxxxx>
- To: <kunal.pisi@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 2 Oct 2007 10:28:10 -0500
Preface: I'm not an EMI expert, but I know some of the basics and know
little-nothing about SATA specifically
EMI is likely caused by common-mode currents of some type. =20
One easy source of common-mode currents is from phase/length skew. If
one vendor has more length skew than the other, that will generate
common-mode currents. Forgive me if I'm saying something you know--
just trying to be helpful. =20
Length skew in the cables/connectors will cause one complement of a diff
pair to rise before the other. This causes the common-mode (i.e.
"average" voltage to deviate from zero) in the switching region. When
the shorter complement rises, the common-mode voltage blips up a little
in the switching region. Then when the shorter complement falls, the
common-mode blips down a little in the switching region. So the
common-mode blip will alternate above zero, below zero, above zero,
below zero... etc. during the switching times. This generates a crude
low-duty cycle "sine-looking" wave at HALF the data rate. In other
words, if you're sending data at 1 Gbps (1 ns period), the common-mode
peak freq will be at 500 MHz. If you see that, that *may* be a
signature of length skew problems. It doesn't take a lot of length
skew to violate emissions. =20
I did one far field CALCULATION earlier (using simplistic Hertzian
dipole assumption-- you can find that equation on the internet), for
example, and I found that 150 mils (~ 4mm) of length skew in a diff pair
(on a 5 meter cable) created a field strength of 90 uV/meter @ a
distance of 10 meters away... violating some FCC specs, depending on the
freq. But the freq of interest is at that half-rate freq.
I've simulated this skew, then analyzed the results in Matlab with an
FFT-- it surprised me to find the half-rate peak in common-mode
current... which is what led me to discover this. =20
Does that makes sense? Anyone else have input? Am I way off base here?
Hope that helps.
- Bart
=20
Bart McCoy Email: McCoy.Bart@xxxxxxxx =09
Mayo Foundation=20
4001 41st Street NW=20
MSC Sn 2-107 Phone: (507) 538-5465
Rochester, MN 55901 Fax: (507) 284-9171
"Fooling around with alternating current is just a waste of time. Nobody
will use it, ever."
Thomas Edison; 1889
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Kunal Sabavat
Sent: Tuesday, October 02, 2007 9:46 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Emission level vary for same type of SATA cables but
different vendors
Hello,
=20
I am seeing different levels of EMI measured values from lab for same
type of SATA cables but from two different vendors. One of the vendors
cable shows much higher EMI values compared with another vendor cable.
measurement setup is same for two vendors cables. I still don't have
full measurement report. these are SATA internal cables, length of the
cable is half meter.
=20
Can someone explain what could be wrong with the cable showing higher
EMI level? Taking cable TDR Impedance scan will help us understand?
=20
--
Kunal
=20
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