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mm precision open
was left in place during all 22 connect/
disconnect measurements; the sample
was cycled at the
; 3 Sigma (ps)
TDR sampling head
only. Likewise during VNA testing, the
sample assembly connection was cycled
at port 1 only. This
not representative of
does introduce a disturbance into the test system such that
the outcome can be observed.
s Fig. 6 ± 3 sigma uncertainty analysis based on measurements of
The number of measurements ( 22)
was determined through a confidence
interval calculation. Twenty-two measurements assure a 98 percent confidence that the sample mean in the
experiment will be within ± 0.08 ps
of the actual population mean. This
is based upon an estimated standard
deviation of 0.16 ps.
VNA Time Delay Measurement
Uncertainty: 3 Sigma Values
Total Measurement Uncertainty: ; 0.08857 ps
; 0.06919 ps
For this portion of the analysis, TDR
and VNA measurement uncertainty
was divided into three categories:
• Instrument uncertainty: Uncertainty associated with the instrument
platform itself, measured through repeat testing.
• Total uncertainty: Uncertainty resulting from the cumulative effects
of instrument characteristics, test fixture, test conditions and operator influences. Measured through connect/
disconnect cycling, includes instrument uncertainty.
• Test uncertainty: Resulting from operator error, test fixture influences and
prevailing environmental conditions at
time of test, measured indirectly.
Figure 6 shows best-case uncertainty for Sample 6. Test uncertainty
values were expected to be similar in
the TDR and VNA due to similarities
in test configurations. With this information, the best-case uncertainty
associated with each instrument platform can be assessed.
The pie graphs in Figure 7 reveal
that 22 percent of the total measurement uncertainty for the VNA is associated with the instrument itself, as
compared to 61 percent for the TDR.
This was a repeating theme throughout
the experiment. This significant difference means that even under ideal test
; 0.1409 ps
; 0.1587 ps
TDR Time Delay Measurement
Uncertainty: 3 Sigma Values
Total Measurement Uncertainty: ;0.2613 ps
s Fig. 7 Total measurement uncertainty
broken down by test and instrument uncertainties.
; 0.01938 ps
conditions, that is minimal test fixture,
operator and environmental influences, the gap in TDR/VNA measurement
uncertainty will remain, as it is inherent to the instrument performance.
Figure 8 compares the 22 connect/
disconnect delta time (Td) delay measurements of Sample 6 relative to the
first measurement using the TDR and
VNA. The VNA measurements have a
range spanning 0.0983 ps as compared
to the TDR’s range of 0.275 ps. Both
data clearly show a trend downward,
that is a progressively shorter device
delay. Although the TDR data suggests a repeatability issue with the 3. 5
mm connector on the TDR sampling
head, it was determined that the variability is associated not with the connector, but the instrument itself.
The downward-trending behavior
noted may be attributed to burnishing
of the SMA/3.5 mm mated interfaces.
A 3. 5 mm connector was used as the
calibrated reference plane to which
the test sample’s SMA was mated.
Connecting and disconnecting the