Re: [PSES] Automotive EMC question

Wed, 13 Jun 2018 06:37:25 -0700

If capacitors do not work, there must be another solution, and it must be known, because it is in use. Perhaps you could dissect your own (or preferably someone else's) means of transport to find out. 

John Woodgate OOO-Own Opinions Only
J M Woodgate and Associates www.woodjohn.uk
Rayleigh, Essex UK

On 2018-06-13 14:26, Ken Javor wrote:
Re: [PSES] Automotive EMC question I don’t doubt that is the rationale.  But what is the fix then for high frequencies where you have stringent RE/RS limits with a shield grounded at one end only?
John Woodgate offered one suggestion using caps at the ungrounded end, but to me that is a commercial fix where you have RE limits imposed at three meters or more above 30 MHz, and RI limits at 1/3/10 V/m, whereas for automotive thee are very stringent RE limits imposed down to 150 kHz and at one meter separation, and also RS limits up to 200 /m. 

Ken Javor
Phone: (256) 650-5261


------------------------------------------------------------------------
*From: *Ted Eckert <000007cf6ebeab9d-dmarc-requ...@ieee.org>
*Reply-To: *Ted Eckert <ted.eck...@microsoft.com>
*Date: *Tue, 12 Jun 2018 22:57:00 +0000
*To: *<EMC-PSTC@LISTSERV.IEEE.ORG>
*Conversation: *[PSES] Automotive EMC question
*Subject: *Re: [PSES] Automotive EMC question
Consider my thoughts as pure conjecture. Automotive electronics often use the chassis as the return for power. For high power devices, you run one wire and tie the device’s ground terminal to the chassis nearby. If the shield was terminated at both ends, would there be a risk of higher return currents flowing on that shield from other devices? 


Ted Eckert
Microsoft Corporation
The opinions expressed are my own and do not necessarily reflect those of my employer or the Society of Automotive Engineers. 


*From:* John Woodgate <j...@woodjohn.uk>
*Sent:* Tuesday, June 12, 2018 2:53 PM
*To:* EMC-PSTC@LISTSERV.IEEE.ORG
*Subject:* Re: [PSES] Automotive EMC question
I hope it also at least recommends that the grounding is at the sending end, so that the cable capacitance is charged from the low-impedance source.  I guess that, e.g. in the auto environment, the risk of large shield currents is too great to allow routine grounding at both ends. But I suppose that grounding via a capacitor at the receiving end is not banned. If possible, this capacitor should be of the lowest possible inductance, which is not difficult with SMD, and if several capacitors are disposed radially around the end of the shield and grounded at their outer ends on a metal ring, the grounding should be good up to at least 1 GHz. 
John Woodgate OOO-Own Opinions Only
J M Woodgate and Associates www.woodjohn.uk <https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.woodjohn.uk&amp;data=02%7C01%7Cted.eckert%40microsoft.com%7C06dcf9fa28b84b06c1c008d5d0b00768%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C1%7C636644376846940054&amp;sdata=L8vMop3oUcGnJh9%2F1N> 
Rayleigh, Essex UK
On 2018-06-12 22:41, Ken Javor wrote:

    The CAN bus spec says that shield(s) are to be grounded at one end
    only. How does this work vs. meeting stringent rf RE and RS
    requirements at frequencies where cables are electrically long?

    Thank you,

    Ken Javor
    Phone: (256) 650-5261


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