Teledyne LeCroy Eye Doctor II - Manual (Page 22)

Eye Doctor II Software
Therefore, it is helpful to follow some simple rules to maximize the effectiveness of using a touchstone
file:
l
Take S-parameter data as low in frequency as the VNA allows to minimize DC extrapolation errors.
l
Take S-parameter data up to one half of the system sample rate. In other words, if you are going
to supply waveforms to the virtual probe at 40 GS/s, it is good to provide data up to 20 GHz. Other-
wise, if the frequency content of the signal provided to the system is much lower, then it is unnec-
essary to sample at a high sample rate anyway.
l
Take S-parameter data with sufficient frequency resolution. The resolution that needs to be pro-
vided depends on the electrical lengths of the circuit elements involved. In other words, if you
have a cable that is 5 ns long, and there are reflections going on in the system, then taking data at
maybe one tenth the minimum frequency resolution of 1/5 ns = 200 MHz (like 20 MHz) is a good
idea. Said differently, in a more practical manner, if there are lots of bumps and wiggles in the S-
parameters that are narrow in frequency, then you will need to measure S-parameter data with an
appropriate frequency resolution to capture these bumps and wiggles. Usually the narrowness of
the bumps and wiggles becomes smaller when the electrical lengths of circuit elements becomes
longer.
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