[SI-LIST] Re: Separating SSO contributed by board vs device


ron miller wrote:

> Hi Fabrizio
>
> I take a slightly different approach to noise on the boards.
>
> You are correct, in my estimation, looking at the power planes 
> coupling noise into
> the sensitive signals and being mixed with the SSO degradation.
>
> However, what appears to random noise is often the non-coherent  
> summation of several
> signals around the board.  PCI, ethernet, serial bus, i2c and any 
> other clocks would be
> included in this catagory.  The coupling means is the return paths, 
> ground or power planes being
> shared by the sensitive signals.
> The first method of determining if the noise might be crosstalk from 
> many frequency sources is to
> put an oscilloscope(differential probe) on signals in the vicintity of 
> the sensitive signal, set the triggering to normal and adjust the 
> trigger level until it triggers about once per second.  Adjusting
> the sweep speed will help to prove it is periodic and to get the peak 
> level(read the trigger level).
>
> Identifying the source is best done using a spectrum analyzer with a 
> small ~ 10pf Cap to keep from
> loading the circuit.  Once the levels of the various  noise sources is 
> measured, adding their power
> levels together(correcting for the loss of the capacitor vs frequency 
> into the 50 ohm scope) the peak
> value that can occur should be similar in magnitude to the peak seen 
> with the oscilloscope adjusted to
> just barely trigger.
>
> SSO noise(board induced component) can be measured directly at the 
> power pins, but the chip
> level SSO can not be directly viewed, and the package is a big part of 
> the reactance generating
> the SSO noise.   Simulation using realistic package models is the only 
> practical method for knowing
> what Simultaneous High-Current Switching at the die, will do to the 
> power rails and how it may
> effect the signal.
> Good hunting.
>
> Ron Miller
>
>
>
>
> Fabrizio Zanella wrote:
>
>> I'm trying to quantify the amount of SSO noise which is contributed by
>> the pc board power delivery system (vias, decoupling, stackup), with the
>> goal of separating this noise from the SSO noise which is contributed by
>> the device and its package.
>> Has anyone attempted to measure this effect, and if so, what would be a
>> good technique? I have two boards available, one with devices populated
>> and the other with some BGAs depopulated, so I have access to the vias.
>> Thanks and regards,
>>
>> Fabrizio Zanella
>> Principal Hardware Design Engineer
>> Broadbus Technologies
>>
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