[SI-LIST] Re: AC series capacitor position in high speeddifferential signals
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>
- Date: Thu, 13 May 2010 14:20:07 -0400
Lee
I don't trust measurements and models that do not agree, either. Which
is why Tom, Al and myself have invested several thousand hours of our
own time into perfecting our modeling and measurement correlation
capabilities.
The problem with measurements that have not been properly de-embedded
(reference plane moved to the DUT) is that what we measure for return
loss (reflections) has been attenuated by 2X the attenuation of the
trace leading into the DUT. The measurement therefore only applies to a
system with exactly the same trace configuration. For example, if one
measures the discontinuity of a DC blocking capacitor with 5" of lead in
trace, the measured return loss (or impedance discontinuity) will be
reduced by the attenuation of the trace at the frequencies of interest.
For mildly low frequencies, trace attenuation is fairly low. But as the
frequency requirements increase, so does the distortion of the
measurement due to the lead-in trace.
The way around this is to either measure very close to the DUT
(capacitor) with microprobes, or to use VNA de-embedding to move the
reference plane as close as possible to the DUT (our preference), ...
and (This is important) utilize known analytical reference structures
for initial correlation. Thus the Beatty standard that can be seen in a
few of our TRL papers. We do this routinely at Teraspeed Consulting to
obtain extremely high quality measurements, which can then be compared
to models, and validated to the accuracy of the instruments. Thus far
we've validated the process with 1% error from DC to 40 GHz.
In my opinion, all of the interesting problems in signal integrity are
return loss (reflection) related, and certainly the placement of DC
blocking capacitors fall into that category. And some of the more
interesting problems often end up being short, rather than long
channels, due to reduced dampening of reflections, and what is termed by
some as "unequalizable energy."
best regards,
Scott
--
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
Lee Ritchey wrote:
> I don't trust either if they don't agree.
>
>
>
>> [Original Message]
>> From: Muranyi, Arpad <Arpad_Muranyi@xxxxxxxxxx>
>> To: <si-list@xxxxxxxxxxxxx>
>> Date: 5/13/2010 10:09:45 AM
>> Subject: [SI-LIST] Re: AC series capacitor position in high
>>
> speeddifferential signals
>
>> Hmmm... I wonder how many of us trusts their measurements
>> (blindly) without validating those? We all know that
>> probing will have an effect on the results, especially at
>> high speeds, right? Should I continue?
>>
>> Arpad
>> =============================================================
>>
>> -----Original Message-----
>> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>> On Behalf Of Lee Ritchey
>> Sent: Thursday, May 13, 2010 11:28 AM
>> To: Istvan Novak
>> Cc: fei xue; si-list@xxxxxxxxxxxxx
>> Subject: [SI-LIST] Re: AC series capacitor position in high
>> speeddifferential signals
>>
>> 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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