[SI-LIST] Waveform Correlation

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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