[SI-LIST] Re: LC filters on SDRAM signal lines
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 24 Jun 2011 12:29:16 -0700
OK so the problem is that you need to protect your receiver from BB
noise. Appropriate placement and stack-up are the first and most
important issues. After that will be noise coupling through the PDN.
Trying to slow your signals down should be a method of last resort.
Steve.
On 6/24/2011 11:08 AM, David Carney (Neenah) wrote:
> 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
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