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From: email@example.com (Jim H)
Subject: Re: Bypass RS-485 shield--ESD problem??
Date: 3 Oct 2002 05:43:09 -0700
NNTP-Posting-Date: 3 Oct 2002 12:43:09 GMT
John Woodgate wrote in message news:...
> I read in sci.electronics.design that Jim H wrote (in
> ) about 'Bypass RS-485
> shield--ESD problem??', on Tue, 1 Oct 2002:
> >I am getting RF leakage into my controller (Maxim 1480B based) from
> >the shield of the signal cable on my 8 node 1200 Meter Rs 485 network.
> > Shield is grounded thru 100 Ohms per datasheet. I will add a ferrite
> >bead at the control end to try to keep the unwanted RF (80-130
> >Mhz)energy out.
> Is this 80 -130 MHz coming from the control equipment or from external
> sources? A series bead may not do much; it's in series with the
> inductance of the shield, which at 100 MHz is very significant. And of
> course you network is many wavelengths long, so there are transmission-
> line effects.
Thanks for responding John- the source of my vhf leakage is the test
lab - we are in the process of compliance testing to EN 61000-4-3 at
> Is your shield at the *control* end connected to the *outside* of the
> enclosure? If it connected inside, it's an antenna and downlead, just
> right for liberating 100 MHz internal stuff that the enclosure is there
> to confine. It's the same at the other nodes; shield to *outside* of
That's what I was afraid of - glad you confirmed it. My shield is
connected to the entrance hub in the steel control enclosure via a
short (1 inch) pigtail INSIDE the enclosure. I have searched for a
grounding hub, with no success.
> > If that doesn't work - I would like to bypass the
> >shield at the other nodes with a 100 nf cap - but won't this become a
> >part of the ESD picture? I need to preserve my 15 KV ESD rating for
> >CE compliance.
> I don't at present understand your reference to ESD. What precisely are
> you concerned about?
Now that I've explained that I'm compliance testing; I assume the
reference becomes clear: EN 61000-4-2 class 3 limits. My concern is
that the remote shield cap has a long enough time constant (with the
100 ohms in the Maxim chip) to accumulate quite a charge when we do
the ESD 2KV discharge directly on the conduit which encloses the RS
485 cable. I realize that the charge will decay rapidly - but may
still fail the tests.
> 100nF is FAR too big for decoupling 100 MHz. Even a 1nF cap needs VERY
> short leads so as not to be resonant in that frequency range. Multilayer
> discoidal capacitors (AVX, for example) behave differently if installed
> correctly. Even a 10 nF has a low impedance up to 1 GHz (I've measured).
> They don't behave as capacitors at very high frequencies; more like
I guess I should have explained initially - I have been assigned the
task of getting a product designed by "digital" engineers through the
compliance testing. I am RF savvy (ham operator for 26 years
-graduated when tubes were the normal active devices, etc, etc.) - but
cleaning things up after a ones and zeros guy is a daunting task.
Challenging, to say the least. At any rate, I'm pleased to have
attracted your attention John - exactly why I posted here.
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