[SI-LIST] Re: Waveform Correlation

Abe et al-
There is another dimension to the model vs. measurement exercise - one which i 
have been grapeling
with for awhile. This is the  time AND frequency domain correlation. Just 
because the time domain
response matches measurement,  does NOT necessarily produce frequency domain 
correlation -
and visa versa. (i.e. transfer response, or TDT in time domain). And it IS  
important to match things
up - mutually.
In my work, i initiate my modeling exercise from TDR measurements because of 
its greater 'bandwidth'
compared to my order of magnitude lower BW VNA, and because i need to see the 
spacial
response. However, when i look at transfer response in the freq domain - i do 
not see as much modeled  loss
as i measure. This is partly to do with my  model construct, for sure.

Would appreciate any commentary regarding this disparity from ALL the community:
- mutual-domain correlation validation practice
-mutual-domain non-correlation reasons

Regards,
Greg Kimball

PS Yes, i also use Excel to marry my PSPICE and measured waveforms

Abe Riazi wrote:

> Dear all,
>
> Comparing simulation generated waveforms with the corresponding lab measured 
> data can yield valuable information towards identifying/eliminating signal 
> integrity problems, enhancing buffer and
> interconnect models, and optimizing simulation/measurement techniques, etc.
>
> One effective correlation approach involves superimposing the measured and 
> simulated
> waveforms utilizing Microsoft Excel. This is feasible because many simulator 
> and measurement
>  tools can output data in a format compatible with Excel. For instance, the 
> Multi-column (.dat) files exported by XNS and  .csv (Comma Space Delimited) 
> files stored by digital oscilloscopes can be readily imported into an Excel 
> spreadsheet.  It is important for the simulated and measured data
> to have similar bit patterns and to be captured at the same location 
> (topology node). Let us
> consider a sample case.
>
> Example.
>  A signal waveform, belonging to memory subsystem of a high-speed 
> motherboard, was captured (in Read cycle) using a HP infinium 1.5GHz  8GSa/S 
> oscilloscope, in conjunction with Tektronix P6248 (1.7 GHz) differential 
> probes. The result was recorded in  .bmp ( for quick viewing) and .csv ( for 
> importing to Excel) formats. Waveform examination revealed that the measured 
> signal initiates
> in High Z state, makes a transition to Low (remains Low for 5 nS), then 
> switches to High (stays
> High for 10 nS), followed by  another transitions to Low (for 5 nS) and 
> finally to High Z.
> The corresponding simulations were performed utilizing XTK. A .mdc ( 
> Multi-Driver
> Contention) file was constructed in a manner that the driver output exhibited 
> same bit pattern as measured signal.  Furthermore, the simulation waveform 
> was probed at the topology node
> consistent with measured data. The simulation result was then exported and 
> saved in Multi-column (.dat) format.   The measured " .csv " and the 
> simulated " .dat " files were entered into Excel for correlation analyses. It 
> was noted that the time axis of simulated result required a conversion (from 
> nano seconds to seconds) and the measured data needed  a time shift of ~ 
> 800pS, in order to optimally overlay the two signals..
>
> Above example has described a way for superimposing measured and simulated 
> waveforms.
>
> With the simulated/measured results properly overlaid, it is possible to 
> evaluate their sameness
> and/or differences. The signal features frequently examined include the low 
> and high DC quiescent voltage levels, the rise/fall times, 
> overshoot/undershoot,  ringing, ringback, glitches, etc.
>
> It is also important that the selected simulation corner (i.e. Min, Typ, Max) 
> and
> the voltage/temperature conditions of measured data to be consistent with 
> each other.
> Although, the nominal corner correlations are very common; correlating under 
> Fast (best case)
> and Slow (worst case) requirements can be also beneficial. Furthermore, it 
> may be necessary to evaluate the signal waveforms at both the driver output 
> and the receiver input pins (or pads).
>
> Comparison of simulated and measured data usually reveals certain differences 
> some
> of which can be due to model deficiencies, simulator limitations, or certain 
> measurement
> inaccuracies.  Correctly interpreting the measured/simulated signal features, 
> their similarities
> and differences constitute a crucial element of accurate correlation.
>
> In summary, comparing key features of simulated and measured waveforms can be 
> helpful
> towards identification/ removal of SI degradation, model optimization, and 
> improvements of simulation/measurement methods. Correlation can be carried 
> out by overlaying the measured
> and simulated waveforms using the Excel program. This approach often 
> necessitates applying
> certain adjustments/shifts to time axis data of one or both waveforms to 
> achieve optimum
> superposition of the results.  It is usually desirable that the points 
> (topology nodes) of capture,
> driver output bit patterns, voltage/temperature conditions of simulated and 
> measured waveforms be nearly matched.
>
> Your comments are highly appreciated.
>
> Respectfully,
>
> Abe Riazi
> ServerWorks
>
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