[SI-LIST] Re: AC series capacitor position in high speeddifferential signals
- From: "Yuriy Shlepnev" <shlepnev@xxxxxxxxxxxxx>
- To: "'Istvan Novak'" <istvan.novak@xxxxxxx>, <leeritchey@xxxxxxxxxxxxx>
- Date: Fri, 14 May 2010 06:34:16 -0700
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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