[SI-LIST] Re: LC filters on SDRAM signal lines
- From: "alfred1520list" <alfred1520list@xxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 24 Jun 2011 11:57:06 -0700
At the risk of stepping outside of my expertise, I'll share my thoughts.
It seems to me that you have options in 3 general areas:
1) Generation: the fast edges are the source of the noise, but they do
have to be the problem. Your options are
1.1) Don't use faster edges than necessary: perhaps your CPU
has edge rate control; perhaps you could use slower SDRAM which
may have slower edge rate. In any case, don't create the noise
in the first place.
1.2) Externally modify the edges, like your filter solutions, or the
ferrite beads solutions
2) coupling: contains the emissions
2.1) Reduce emission/coupling efficiency: like others have suggested,
find out how/what the coupling mechanism is. Is it radiating
through trace impedance discontinuity? via? broken return path?
Can you change trace impedance to reduce emission? Is it
leaking through the PDN?
2.2) As others have suggested too, your last option is shielding.
3) How does it get into the GSM module?
Is it through the antenna or some where else? If through the
antenna, can you move the antenna? If some where else,
you might improve the isolation.
I'm not familar enough with GSM. You saw -90 dBm noise.
How much lower do you need? If it is 10 dBm or less you may
not have a very hard problem.
Good luck.
Best Regards,
Alfred Lee
----- Original Message -----
From: "David Carney (Neenah)" <David.Carney@xxxxxxxxxx>
To: "steve weir" <weirsi@xxxxxxxxxx>
Cc: <si-list@xxxxxxxxxxxxx>
Sent: Friday, June 24, 2011 11:08 AM
Subject: [SI-LIST] Re: LC filters on SDRAM signal lines
> 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
>>
>>
>>
>>
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>
>
> --
> Steve Weir
> IPBLOX, LLC
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