[SI-LIST] Re: AC series capacitor position in highspeeddifferential signals
- From: Istvan Novak <istvan.novak@xxxxxxx>
- To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
- Date: Sat, 15 May 2010 13:03:08 -0400
Valid request. It looks to me that this will trigger some interesting
DesignCon papers... :-)
Lee Ritchey wrote:
> I'd like to see actual measurements that show this degradation before I
> sign up.
>
>
>
>> [Original Message]
>> From: Istvan Novak <istvan.novak@xxxxxxx>
>> To: <leeritchey@xxxxxxxxxxxxx>
>> Cc: fei xue <harrison_cls@xxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
>> Date: 5/13/2010 6:05:12 PM
>> Subject: [SI-LIST] Re: AC series capacitor position in
>>
> highspeeddifferential 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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