[SI-LIST] Re: LC filters on SDRAM signal lines
- From: "David Carney (Neenah)" <David.Carney@xxxxxxxxxx>
- To: "steve weir" <weirsi@xxxxxxxxxx>
- Date: Fri, 24 Jun 2011 13:08:14 -0500
To clarify, our problem is that the GSM module is picking up broadband
noise whose source is the processor SDRAM bus on the board that the GSM
module is attached to. The proposal is to filter the SDRAM data lines
to reduce the high frequency 869-894 MHz + content in these signals
without affecting the useful frequency content in these 75 MHz signals
and wrecking the signal integrity or timing on the SDRAM bus. Reducing
this high frequency content should reduce the amount of noise coupled in
to the GSM receiver which is currently causing it to have a higher than
acceptable BER.
This is not necessarily a desirable solution to us since adding all
these filters is a challenge for space and redoing a lot of routing, but
it has been suggested that this is one of the most effective solutions.
I'm trying to figure out how to evaluate whether it will be effective
and also whether it will not wreck the SDRAM bus SI and timing.
It sounds like your suggestion is to look more closely at the PDN around
the SDRAM bus before considering filtering the data lines?
Thanks for the response.
-----Original Message-----
From: steve weir [mailto:weirsi@xxxxxxxxxx]
Sent: Friday, June 24, 2011 12:19 PM
To: David Carney (Neenah)
Cc: si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] LC filters on SDRAM signal lines
The point of any filter is to separate signal energy from noise energy.
Before you can begin thinking about a filter you need to determine if
the offending noise is well outside your signal spectrum. If it is not,
then your only available option is to shield. That is assuming that
your problem is GSM pick-up on the signal lines. Even though your SDRAM
is only running a 75MHz clock, in order for the timing to be stable you
will have spectrum at least out to around 400MHz, which is close to half
of the GSM. You don't have much of a window between your required pass
and stop bands.
The GSM "buzz of death" can get directly into anything that acts as an
antenna. It also does bad things to PDNs. You need to have a hard look
at your PDN and your layout to see just where it is that the GSM noise
is coupling in. If you are lucky, you can fix this with some changes to
your bypass network. If you've got a poorly designed stack-up and
layout where GSM noise is for example directly coupling into your signal
returns, then you will have to address that.
Steve.
On 6/24/2011 8:47 AM, David Carney (Neenah) wrote:
> I'm working on an embedded product with a GSM cellular module. The
> module is picking up broadband noise in the GSM850 band (869-894 MHz)
at
> levels of approximately -90dBm to -100dBm from the board it is on.
The
> noise is causing BER levels that are too high (fails some tests). The
> source of the noise has been tracked to a processor SDRAM bus
interface
> running at 75 MHz clock frequency. This processor and SDRAM are
> physically close to the GSM module on the board. One of the many
> suggestions for mitigating this problem that we are considering is to
> add LC filters to all of the SDRAM data lines such as the following
> example parts:
>
>
> Murata NFM18PS105R0J3
>
(http://search.murata.co.jp/Ceramy/image/img/PDF/ENG/L0111S0111NFM18PS.p
> df)
>
> TDK MEA1608PH (http://www.tdk.co.jp/tefe02/e9621_mea_signal_1.pdf)
>
>
>
> These parts are recommended for LCD interfaces. Does anyone have
> experience using these on SDRAM interfaces? Is this an effective way
to
> solve the problem? What considerations do we need to account for in
> using them? Has anyone modeled these parts for signal integrity
> simulations? We have been told by Murata that no model is available.
> What modeling approach did you use?
>
>
>
> Thanks.
>
>
>
>
>
> David T. Carney P.E.
>
> Senior Design Engineer
>
> Plexus Engineering Solutions
>
> Neenah Design Center
>
> 920.751.5646
>
>
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
>
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV 89501
www.ipblox.com
(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
