Teledyne LeCroy Eye Doctor II - Manual (Page 21)

Operator's Manual
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Lines beginning with ! are comment lines. Comments must be at the front of the file. No com-
ments are allowed once the frequencies and S-parameter values start.
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There should be at least one line that begins with ‘#'. This line is a line that contains tokens that
help interpret the S-parameters. Valid tokens are:
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Hz – frequencies are in Hz.
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MHz – frequencies are in MHz.
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GHz – frequencies are in GHz.
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MA – S-parameters are in magnitude/angle form where the magnitudes are true mag-
nitudes and the angle is in degrees.
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RI – S-parameters are in real/imaginary form.
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DB – S-parameters are in magnitude/angle form where the magnitude is in decibels and the
angle is in degrees.
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If the above tokens are not found, the frequencies are assumed to be in MHz, and the S-param-
eters are assumed to be in magnitude/angle form.
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The S-parameters for each frequency are assumed to be listed as:
[frequency] (S11) (S12) …(S1[N])(S21) (S22) … (S2[N]) … (S[N]1) (S[N]2) … (S[N][N])
where N is the number of ports dictated by the file extension, except for two-port S-parameters
which are assumed to be listed as [frequency] (S11) (S21) (S12) (S22) according to the standard.
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All S-parameters are assumed to be 50 Ohm. In S-parameter files, on the line beginning with ‘#',
usually there is a token pair: R [Z0] where [Z0] is the characteristic impedance. We assume Z0 is 50
and ignore these tokens.
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All frequencies are assumed to go from the first frequency listed to the last frequency listed with
constant frequency spacing.
N OTES A BOUT S- PARAMETER F ILE I NTERPRETATION A ND U SAGE
S-parameters provided to the virtual probe will tend to have various frequency scales, differing in both
resolution (frequency spacing) and range (start frequency and end frequency). The virtual probe com-
ponent will resample all S-parameter sets onto a new frequency scale dictated by the system sample rate
(half the sample rate will be the last frequency) and the time length (see Time Length) – where the time
length dictates the new frequency resolution.
In performing this resampling, it is important to understand that the resolution may be higher or lower
than the original resolution and that the frequency range may require extrapolation of points. When the
frequency range is higher than that provided in the S-parameter data (i.e. the S-parameters are not pro-
vided to half the system sample rate), then the S-parameter data will be set to zero at these extra
required data points. Furthermore, most S-parameter data does not go to DC, because VNAs don't meas-
ure data to DC, and therefore the DC response is also extrapolated by setting the low frequency points
equal to the first frequency provided in the S-parameter data.
922961 Rev A 19