[SI-LIST] Re: LED with series bead
- From: Eddy <eddyvk@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Sat, 10 Nov 2007 13:25:01 -0800 (PST)
I cannot help but to feed it some more:
OK, so the LED responds in 50ns.... I think this means it can fully turn on or
off in this amount of time. A faster signal may not fully turn it on and off
but it would still make it 'flicker' a bit, right? The slow LED may attenuate
but does it fully extinguish the signal? ;-)
If there is a resistor in series with the LED you could also place a capacitor
parallel to the LED to kill the data. If there is no resistor then maybe a
series diode (schottky) can be used together with the capacitor although this
might get a bit expensive....
So far poking fun at this subject. A more serious approach is to look at the
business side of it. Could you sell more equipment or for a higher price if you
could claim "secure status LED's"??? I remember laughing very hard in 1999 at
an ad for Y2K compliant crystal oscillators (I was designing crystal
oscillators at the time). Then I realized that loads of purchasing managers
would probably fall for this argument, especially in 1999..... Paranoia can be
good business.
One more thought: I deal a lot with customers that want to see every little
feature in our designs characterized and verified to be reliable. How are you
going to provide the paranoid customers with proof that the beads are really
effective at blocking the data....? Would you need to provide a spectrum plot
of the optical modulation while transmitting data? ;-)
Eddy
www.phaselink.com
----- Original Message ----
From: "olaney@xxxxxxxx" <olaney@xxxxxxxx>
To: john.zelno@xxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Sent: Friday, November 9, 2007 8:47:12 AM
Subject: [SI-LIST] Re: LED with series bead
Paranoid indeed, as the referenced paper speaks to the case of RS-232
data directly modulating a status LED. Those rates are so slow that a
bead would do no more than slightly round the edges of the pulses, and
is
useless. For serdes serial speeds, the LED response is too slow
(around
50 nsec for garden variety indicator LEDs that I have bothered to
check)
to respond to individual bits. In any instance, if you have control of
the design, then it is simple to drive the LED with something other
than
raw data. The use of beads sounds like a fix dictated by a manager.
Am
I right?
Orin
On Fri, 9 Nov 2007 07:07:22 -0800 "Zelno, John" <john.zelno@xxxxxxxxxx>
writes:
> We have a very paranoid customer, and have put beads on our LEDs to
> prevent serial data that may have coupled onto the LED signals from
> being modulated by the LED, and thus able to be picked up by a photo
> detector. Much of the paranoia was based on some studies done (for
> 1 example, see http://applied-math.org/optical_tempest.pdf).
>
> -John Zelno
> ________________________________
>
> From: si-list-bounce@xxxxxxxxxxxxx on behalf of Don Nelson
> Sent: Thu 11/8/2007 6:06 AM
> To: zhuyongfa@xxxxxxxxxx
> Cc: steve weir; vijayanand@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx;
> lihuafu@xxxxxxxxxx; duyumin@xxxxxxxxxx
> Subject: [SI-LIST] Re: LED with series bead
>
>
>
> I was once instucted to add ferrite beads to the front panel LEDs of
> a GigE linecard I was developing to prevent EMI (we already tested
> the card and passed EMI, so I was not pleased about this request).
> The LEDs in question were Link/Activity LEDs driven directly by the
> GigE PHY.
>
> When the cards came back from the factory and I began testing them,
> they were a complete disaster... Every once in a while, the PHY
> would go haywire and I'd need to reconfigure it. After a long night
> if pulling my hair out I realized that any time one of these LEDs
> switched from ON to OFF (such as pulling a fiber, causing the LINK
> LED to extinguish), the PHY got zapped; the ferrite caused a
> back-EMF into the PHY when the current tried to switch
> instantaneously from 15mA to 0.
>
> I replaced the ferrites with zerohms and the product worked
> perfectly -- and also passed EMC!
>
> -d
> --
> Don Nelson
> Ericsson
> Warrendale, (Pittsburgh) PA, USA
>
> "There are 10 kinds of people in this world: those who can count in
> binary and those who can't."
>
>
> On Wednesday, November 07, 2007, at 11:19PM, "z46147"
> <zhuyongfa@xxxxxxxxxx> wrote:
> >Content-type: text/plain; charset=iso-8859-1
> >Content-transfer-encoding: 7BIT
> >Steve,
> >
> >Agree with ESD susceptibility if LED applied to front panel.
> >
> >Thanks and regards.
> >
> >
> >Zhuyongfa
> >HUAWEI TECHNOLOGIES CO.,LTD.
> >
> >
> >Address: Huawei Industrial Base
> >Bantian Longgang
> >Shenzhen 518129, P.R.China
> >Tel:+86-755-89653025
> >Fax: +86-755-89650731
> >E-mail: zhuyongfa@xxxxxxxxxx
> >www.huawei.com
>
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> > ----- Original Message -----
> > From: steve weir
> > To: zhuyongfa@xxxxxxxxxx
> > Cc: vijayanand@xxxxxxxxxxxxx ; si-list@xxxxxxxxxxxxx ;
> lihuafu@xxxxxxxxxx ; duyumin@xxxxxxxxxx
> > Sent: Thursday, November 08, 2007 11:46 AM
> > Subject: Re: [SI-LIST] Re: LED with series bead
> >
> >
> > Beads sometimes get applied to front panel LEDs, switches and
> such in
> > the belief that this will limit EMI radiation and/or ESD
> > susceptibility. You can decide for yourself if you think, there
> is a
> > lot that a ferrite bead is going to do that a 100-300 Ohm current
> limit
> > resistor doesn't already do.
> >
> > Steve.
> >
> >
> > z46147 wrote:
> > > Content-type: text/plain; charset=gb2312
> > > Content-transfer-encoding: 7BIT
> > > Hi all,
> > >
> > > I also want to know root cause of this application.
> > >
> > > Any thought on it would be appreciated.
> > >
> > > Best regards.
> > >
> > > Zhuyongfa
> > > HUAWEI TECHNOLOGIES CO.,LTD.
> > >
> > >
> > > Address: Huawei Industrial Base
> > > Bantian Longgang
> > > Shenzhen 518129, P.R.China
> > > Tel:+86-755-89653025
> > > Fax: +86-755-89650731
> > > E-mail: zhuyongfa@xxxxxxxxxx
> > > www.huawei.com
> > >
>
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> > > ----- Original Message -----
> > > From: Vijay Anand
> > > To: si-list@xxxxxxxxxxxxx
> > > Sent: Tuesday, November 06, 2007 6:30 PM
> > > Subject: [SI-LIST] LED with series bead
> > >
> > >
> > > Dear All,
> > > In our design we connect a bead in series with LED while
> driving from an I/O pin. I dont know why this bead is connected (
> still series resistor is there for current limiting). Is there is
> any relation with EMI? Kindly do the needful.
> > > With Regards,
> > > Vijay Anand. G
> > >
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