[SI-LIST] Re: AC series capacitor position in high speeddifferential signals
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Yuriy Shlepnev" <shlepnev@xxxxxxxxxxxxx>, "Istvan Novak" <istvan.novak@xxxxxxx>
- Date: Sat, 15 May 2010 08:38:38 -0700
My measurements using 0402 capacitors and 12 mil drilled vias on both sides
of the capacitor in a 100 mil thick PCB, which is a common thickness, shows
that there is a very small reflection off the capacitance of the via
itself, but the added capacitance of the capacitor mounting pad is quite
small and much less than the capacitance of the plated through hole. This
is an easy calculation.
Putting the same date in a simulator results in pretty much the same result.
The reflection from the combination is not zero, but is quite small and has
not shown up as significant degradation of the signal, either by simulation
or measurements. I'd be interested in what the configuration was that
resulted in 20% reduction.
> [Original Message]
> From: Yuriy Shlepnev <shlepnev@xxxxxxxxxxxxx>
> To: Istvan Novak <istvan.novak@xxxxxxx>; <leeritchey@xxxxxxxxxxxxx>
> Cc: fei xue <harrison_cls@xxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> Date: 5/14/2010 6:42:53 AM
> Subject: [SI-LIST] Re: AC series capacitor position in high
speeddifferential signals
>
> Istvan and Lee,
> I guess the next step in resolving the contradiction would be validation
of
> your measurements with a simulation that captures all relevant details of
> the problem :)
> For what it worth, I simulated the effect of the capacitor placement in a
> simple channel - the results and some observations on the subject are in
App
> Note #2010_02 at http://www.simberian.com/AppNotes.php. As expected,
> swapping ideal driver and receiver does not change the transmission at all
> due to the reciprocity principle (the statement is valid for any linear
> reciprocal system). Though the reflection strongly depends on the location
> of the capacitor and may affect the overall signal transmission due to
> non-linearity and reflections from the driver and receiver.
>
> Best regards,
> Yuriy Shlepnev
> www.simberian.com
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On
> Behalf Of Istvan Novak
> Sent: Thursday, May 13, 2010 6:02 PM
> To: leeritchey@xxxxxxxxxxxxx
> Cc: fei xue; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: AC series capacitor position in high
> speeddifferential signals
>
> Lee,
>
> I agree with you that people should not blindly trust only simulation
> results. Those who follow my work
> know that for me a question answered means that measurements,
> simulations and understanding all have
> to agree to a sufficient degree. That having said I cant argue with
> your statement when you say that your
> measurements show negligible reflections in your boards: I trust it is
> true for the boards you built.
>
> What prompted my posting was that your original comment could be
> interpreted as a suggestion that
> the capacitor placement makes no difference under any circumstances as
> long as the channel is linear.
> My point was that there are situations (and we dont need to think about
> something exotic, just cases
> when pad discontinuity and/or via stub discontinuities are not removed
> for any reason) when the location
> of capacitor makes a difference in the received eye parameters by a
> noticeable way. With very typical
> parameters we can end up having 15-20% fluctuation of eye closure
> horizontally and vertically as we
> slide the capacitor along the channel. This can be simulated, but can
> also be measured on real system
> channels.
>
> Regards,
>
> Istvan Novak
> Oracle
>
>
>
> Lee Ritchey wrote:
> > I would maintain from my measurements that the reflections are
negligibly
> > small.
> >
> > I would also maintain that simulation results, no matter what the tool
or
> > operator, should not be trusted until they are validated by measurements
> to
> > insure the modelling is accurate. I've seen too many simulations done
> > incorrectly which were used to develop design rules that were either
> > ineffective or caused problems that I, for one, will not use the results
> of
> > unvalidated simulations.
> >
> > Each engineer can make up his or her own mind on this, but my experience
> > says validate simulation results before betting any money on them.
> >
> > I'm not trying to offend anyone who does simulations, just advising
> proceed
> > with caution.
> >
> > Lee
> >
> >
> >
> >> [Original Message]
> >> From: Istvan Novak <istvan.novak@xxxxxxx>
> >> To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
> >> Cc: fei xue <harrison_cls@xxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> >> Date: 5/12/2010 8:55:32 PM
> >> Subject: Re: [SI-LIST] Re: AC series capacitor position in high
> >>
> > speeddifferential signals
> >
> >> Lee,
> >>
> >> As it was pointed out in earlier threads, location does make a
> >> difference unless reflections are
> >> negligibly small. Assuming linearity, when we move components around,
> >> reciprocity prevails,
> >> but voltage transfer function from source to load will change, which
in
> >> turn impacts eye
> >> parameters.
> >>
> >> Regards,
> >>
> >> Istvan Novak
> >> Oracle-America
> >>
> >>
> >> Lee Ritchey wrote:
> >>
> >>> This has been answered before on this forum. Since the circuits are
> >>> linear, it does not matter from a signal integrity point of view.
> >>>
> >>> Lee
> >>>
> >>>
> >>>
> >>>
> >>>> [Original Message]
> >>>> From: fei xue <harrison_cls@xxxxxxxxxxxx>
> >>>> To: <si-list@xxxxxxxxxxxxx>
> >>>> Date: 5/12/2010 11:35:49 AM
> >>>> Subject: [SI-LIST] AC series capacitor position in high speed
> >>>>
> >>>>
> >>> differential signals
> >>>
> >>>
> >>>> Hello all,
> >>>> We often can get different guideline of placing capacitor position
when
> >>>>
> >>>>
> >>> placing AC series capacitor on high speed differential signals, like
> >>>
> > PCIe,
> >
> >>> SAS or LVDS signals. sometimes we followed the guideline to put
> >>>
> > capacitors
> >
> >>> near driver, sometimes near multi-connection connectors or sometimes
> >>>
> > put it
> >
> >>> near receivers.
> >>>
> >>>
> >>>> Could anybody tell me what is the consideration of capacitor placing
> >>>>
> >>>>
> >>> position? Thanks!
> >>>
> >>>
> >>>> Harrison
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >
> >
> >
> >
>
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