FreeLists / si-list / [SI-LIST] Re: How to test the power ripple ?

[steve weir <weirsi@xxxxxxxxxx>]

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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