[SI-LIST] Re: AC series capacitor position in high speeddifferential signals
- From: Istvan Novak <istvan.novak@xxxxxxx>
- To: Chris Cheng <Chris.Cheng@xxxxxxxx>
- Date: Sun, 16 May 2010 10:03:10 -0400
Chris,
Any combination of linear resistors and linear reactances would still be
linear. Any voltage
dependence in any of them would make the combination nonlinear. Though
when I mentioned
receiver nonlinearity in my earlier posting, I had more drastic versions
in mind, when
something gets driven into saturation.
Last night I sent an off-line message to Yuriy to clarify further what I
meant by transfer
function...
Best Regards,
Istvan Novak
Oracle-America
Chris Cheng wrote:
> Yuriy,
> Do we still consider the end points linear if the transeivers are combination
> of parasitics capactiors (due to ESD structures) and resistive terminator (tx
> and rx circuits) ? In particular, the ESD capacitance is voltage dependent ?
> Thanks in advanced,
> Chris
>
> ________________________________________
> From: si-list-bounce@xxxxxxxxxxxxx [si-list-bounce@xxxxxxxxxxxxx] On Behalf
> Of Yuriy Shlepnev [shlepnev@xxxxxxxxxxxxx]
> Sent: Saturday, May 15, 2010 1:15 PM
> To: 'Istvan Novak'
> Cc: leeritchey@xxxxxxxxxxxxx; 'fei xue'; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: AC series capacitor position in high speeddifferential
> signals
>
> Istvan,
>
> Lorentz theorem states that "the response of a system constructed of linear
> reciprocal materials to a source is unchanged when source and measurer are
> interchanged". It means that the transfer function from a source with 75-Ohm
> terminator to a 25-Ohm load for instance is exactly the same as the transfer
> in the opposite direction. I guess you meant that the transfer function will
> be different if you swap the termination impedances - it is so, but still
> identical in both directions. It might be counterintuitive and if the
> differences in the transfer functions or in the transmission parameters are
> observed - for PCB applications it means either non-linearity of some
> elements or errors in measurements or computations (as far as I know
> non-reciprocal materials such as biased ferrites are not used for PCBs and
> packaging).
> I have added a few more illustration for a case of mismatched source and
> load and for a case with asymmetric resonances to the app note at
> http://www.simberian.com/AppNotes/AC_CouplingCapacitors_2010_02.pdf (see the
> backup slides).
>
> Best regards,
> Yuriy
> www.simberian.com
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of Istvan Novak
> Sent: Friday, May 14, 2010 9:16 PM
> To: Yuriy Shlepnev
> Cc: leeritchey@xxxxxxxxxxxxx; 'fei xue'; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: AC series capacitor position in high
> speeddifferential signals
>
> Yuriy,
>
> Nice app note, this will be useful for people to comprehend reciprocity
> and symmetry. With Simbeor
> one can also model the via transitions along the path. The slides
> nicely illustrate the point that in a linear
> but asymmetrical channel the transfer parameters are the same from
> either end IF there are no further
> reflections. However, if in a next step you also simulate the
> source-to-load voltage transfer function with
> slightly mismatched transmit and receive impedances, the transfer
> function and the eye will be direction
> dependent even if everything is linear.
>
> Best regards,
> Istvan
>
>
>
>
> Yuriy Shlepnev wrote:
>
>> 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
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>
>>>
>>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
>
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
>
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
> This email and any attachments thereto may contain private, confidential, and
> privileged material for the sole use of the intended recipient. Any review,
> copying, or distribution of this email (or any attachments) by others is
> strictly prohibited. If you are not the intended recipient, please contact
> the sender immediately and permanently delete the original and any copies of
> this email and any attachments thereto.
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:
> //www.freelists.org/archives/si-list
>
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
