[SI-LIST] Re: About the chassis gnd and logic gnd

Peter, my best recommendation is frequent ties between logic and shelf 
ground.  Keep the -48V return fully isolated from logic and earth within 
each chassis.  The -48V return should tie back to earth at the customer 
battery plant / power supply.
Your backplane attachment sounds OK, at least superficially.  There might 
be some issues if this is a midplane or you are plugging in transceivers to 
the outside world.

Are you developing that in China or CA?  If there is someone in Alameda 
involved we could set up an hour or two consulting session to review the 
plans.

Regards,


Steve.
At 10:25 AM 3/26/2004 +0800, peter zhu wrote:
>Steve:
>
>We are developping ATCA chassis and using isolated DC-DC converters.
>In fact, In ATCA chassis, there are three gnd: shelf gnd, -48v Return, and 
>logic ground. Now I plan to tie the shelf gnd and logic gnd on backpalne, 
>and let the -48V Return separated. The 48V Return may tie to shelf gnd on 
>shelf using single point.
>
>On backpalne PCB, we only use one gnd stack, all logic gnd and shelf gnd 
>from connector pins are connected to this gnd stack. if the screw that fix 
>the backpalne on the margin are installed, the gnd stack will be connected 
>to shelf. do you think this is feasible?
>
>Thanks!
>
>Peter
>----- Original Message -----
>From: <mailto:weirsp@xxxxxxxxxx>steve weir
>To: <mailto:peter.zhu@xxxxxxxxxx>peter.zhu@xxxxxxxxxx ; 
><mailto:si-list@xxxxxxxxxxxxx>si-list@xxxxxxxxxxxxx
>Sent: Thursday, March 25, 2004 11:52 PM
>Subject: Re: [SI-LIST] About the chassis gnd and logic gnd
>
>Peter, the subject has been covered many times.  There are seemingly 
>religious views on this.
>
>I belong to the "reduce antenna gain" camp, which is to say that at high 
>frequencies, even an air gap does not effect isolation, it just increases 
>antenna gain and ESD susceptibility.  So multipoint bonding is the 
>preferred strategy in my book.  Bond often, and bond well.  There is one 
>caveat, and that is if you have external low voltage power supplies, then 
>you have to watch the return current path.  But since you likely use 
>isolated DC-DC converters, this should not present an issue to you.
>
>This point of view is supported by practices at major successful 
>manufacturers such as Cisco where those practices resulted in behemoth 
>boxes like the 12000 that have no EMC issues at all.  It is also supported 
>by respected authors like Mark Montrose, and Doug Smith who show the 
>science behind why this practice works, as well as successful engineers 
>with many years experience like Chris Cheng.  My recollection is that John 
>Barnes covers this subject in his books as well.  If you can afford them, 
>and even if you can't, John brings many years experience including with 
>ultra cost sensitive consumer product.
>
>If you go to Doug Smith's web site, www.emcesd.com and look up his April 
>2002, May 2002, and October 2002 "Technical Tidbits" he demonstrates the 
>value of multipoint grounding very clearly for ESD immunity.
>
>Regards,
>
>
>
>Steve.
>At 07:43 PM 3/25/2004 +0800, peter zhu wrote:
>>all:
>>About the chassis GND and the logic GND, some company tie them together, and
>>some sompanies separate them.
>>In telecom equipments, I see many companies, such as NOKIA, tie them
>>together, and the backplane only has uniform GND layer for chassis gnd and
>>logic gnd. We separate them in our backplane, but it will result in PCB gnd
>>layer partition. So form the PCB view, we hope the uniform gnd.
>>What's the recommendation form the EMC/EMI view? and what's your company's
>>strategy?
>>
>>Thank,
>>
>>Peter
>>UTStarcom
>>
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>Steve Weir
>Teraspeed Consulting Group LLC
>2926 SE Yamhill St.
>Portland, OR 97214
>(503) 239-5536
>http://www.teraspeed.com
>------------------------------------------------------------------------------------------------------
> 
>
>Teraspeed(SM) is the service mark of Teraspeed Consulting Group LLC

Steve Weir
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com
------------------------------------------------------------------------------------------------------
Teraspeed(SM) is the service mark of Teraspeed Consulting Group LLC


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