[SI-LIST] Re: How to test the power ripple ?

Note, should have said:

From a power delivery standpoint only, you care about PDN noise up to 
about 2-3X the cut-off frequency of any given IC package as mounted on your 
PCB.  


steve weir wrote:
> Whoever is saying such things, hasn't been saying them to me.  I think 
> you should pose your question to them.
>
> If your concern is noise transmission through the PDN, this varies 
> substantially depending on where you look and what excites the PDN.  
>  From a power delivery standpoint only, you care about PDN noise up to 
> the cut-off frequency of any given IC package as mounted on your PCB.  
> This can be as low as a couple of MHz, or as high as hundreds of MHz 
> depending on the power supply and IC of interest.  A core logic power 
> connection may well be oblivous to anything above 10 or 20MHz, while I/O 
> can be sensitive to 100's of MHz.  It all depends on the specifics of 
> the IC.
>
> Many power supply manufacturers specify noise and ripple as measured 
> through a 20MHz single pole LPF.  It's convenient for them, but as 
> Istvan Novak has pointed-out, allows some really ugly supply outputs to 
> advertise much cleaner looking specifications.  If this is a concern, 
> and you are not in a position to extract / impose a wideband 
> specification from your suppliers then you can either: periodically test 
> and qualify incoming product, or incorporate a 20MHz LPF into your 
> design. The former approach has the disadvantage of what to do with the 
> nonconforming supplies.  As far as the vendor is concerned the supplies 
> work to spec and they have no reason to take them back.  You also have 
> to worry about what you are going to use to get product out the door.
>
> The latter approach has the advantage that you can rely on the 
> manufacturer specs as-is. Often this filter function can be realized 
> with a single low inductance capacitor like an X2Y(r) mounted close to 
> the power supply connection to the PCB, and attention to local etch.  A 
> further advantage of this type of approach is that the filter is 
> bidirectional.  The filter attenuates high frequency load noise feeding 
> through back to the power supply input where in many cases it is a 
> potential EMC issue.
>
> Steve
>
> icer world wrote:
>   
>> hello all:
>> When  test the ripple of the power of a board ,It's said that I should test 
>> it below 20MHz.But I found that it's much defferent when I test the ripple  
>> in the full band width frequency.As I think we should test the ripple 
>> without any frequency limit ,or we can't observe the noise produced by other 
>> devices which can couple into the power system .I hope someone will give me 
>> some !advice,thanks
>>
>>       
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>   


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