[SI-LIST] Re: PCIe DCD / Jitter simulation
- From: Hermann Ruckerbauer <hermann.ruckerbauer@xxxxxxxxxxxxx>
- To: steve weir <weirsi@xxxxxxxxxx>
- Date: Mon, 26 Oct 2009 21:12:06 +0100
Hi Steve,
thanks for the response, but I'm not sure if I understand, but I guess
you do have the same understanding as I do:
Different to the Words "Duty Cycle Distortion" which originally was used
e. g. for a clock pattern with different high and low periods (as
described by Joseph from LeCroy) the spec tries to accont for the
minimum Eye that is caused from different effects:
- ISI due to Bandwidth limitation in the Transmitter (driver + package)
- SSO noise caused by Power Delivery
- X-talk (e. g. from the package)
- DCD from the internal clock tree
- maybe even external noise from switching Power supplys
So in this case a simulation with a DCD of 0.1UI with no jitter
distribution would be enough to simulate. But now it depends on the
implementation of the DCD in the simulator.
If e. g. always the first bit is the larger one and the second is the
smaller one the pattern ideally should run twice in such a way, that
each bit hits the small eye once.
Please let me know if this is what you ment.
regards
Hermann
EKH - EyeKnowHow
Hermann Ruckerbauer
www.eyeknowhow.de
hermann.ruckerbauer@xxxxxxxxxxxxx
Veilchenstrasse 1
94554 Moos
Tel.: +49 (0)9938 / 902 083
Mobile: +49 (0)176 / 787 787 77
Fax: +49 (0)3212 / 121 9008
steve weir schrieb:
> Hermann, frequency dependent loss in the signal band will give you
> pattern dependent distortion, IE DCD.
>
> Steve
> Hermann Ruckerbauer wrote:
>> Hello,
>>
>> I'm just looking in PCI express gen2 simulations, but do not fully
>> understand the Spec regarding DCD and Jitter (low and high frequency)
>> for worst case simulations.
>>
>> There is mentioned, that the simulation should be done with 0.1 DCD.
>> Additionally there are 78ps Dj and 28ps for slewing the eye.
>>
>> 0.15UI is the max. High Frequency TX jitter. From this 30ps (0.15UI)
>> one should include 0.1UI (20ps) as DCD in the simulation.
>> ==> What is the Jitter distribution, that should be assumed here?
>> I guess a fixed 0.1UI DCD is not correct. Personally I would use +-
>> 0.05UI with some gaussian distribution, but I'm not sure if this is the
>> intension there.
>> This would reduce the eye size, but still results in a nice symmetric
>> eye.
>> So in my understanding this is not really a Duty cylce distortion with
>> e.g. each even bit short and each odd bit long, but high frequency
>> jitter e. g. introduced by SSO noise and package x-talk.
>> ==> Is my understanding correct ?
>>
>> Additionally there is a value of 78ps Dj mentioned. not sure if I need
>> to add this in simulation or data evaluation, or if this is accounted
>> for by the DCD already.
>> ==> My understanding is, that this is the max value of the sum of the
>> 0.1DCD + the Jitter introduced by the channel.
>> Is this correct, or do I need to add this DJ in addition (in simulation
>> or data evaluation)?
>> I can evaluate this, but if I place a Receiver eye mask it should be
>> enough not to violate the mask, correct ?
>>
>>
>> Finally there is there is the jitter that I need to add in the
>> dataevaluation by slewing the eye (e. g. 28.9ps for common clock
>> architecture.
>> ==> is it OK to expand the eye mask by this number (+-15ps from the
>> center) ?
>>
>> So far i have not seen any documents that describe it in a way that I
>> would understand it (guess I'm just to stupid). Maybe somebody can
>> enlighten me here, or point me to some description.
>>
>> Thanks and regards
>>
>> Hermann
>>
>>
>>
>>
>> EKH - EyeKnowHow
>> Hermann Ruckerbauer
>> www.eyeknowhow.de
>> hermann.ruckerbauer@xxxxxxxxxxxxx
>> Veilchenstrasse 1
>> 94554 Moos
>> Tel.: +49 (0)9938 / 902 083
>> Mobile: +49 (0)176 / 787 787 77
>> Fax: +49 (0)3212 / 121 9008
>>
>>
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