s Fig. 8 Twenty-two connect/disconnect
measurements in sequence.
; Td (ps)
–0.25 22 20 18 16 14 12
s Fig. 9 Twenty-two connect/disconnect
measurements of Sample 6 in sequence.
SMA interface in succession (without
cleaning between cycles, as was done
in the experiment) has the potential to
burnish the mated connector interface
components. It was theorized that over
the course of 22 test cycles, the mated
interfaces were sufficiently abraded to
experience improved electrical contact, as evidenced by a reduction in insertion loss and electrical length.
It is of some interest to compare the
absolute time delay values for Sample 6
as measured by the TDR and VNA. An
examination of repeat testing produced
an average time delay of 0.817364 ns
for the VNA and 0.849754 ns for the
TDR; a difference of 32. 5 ps. This discrepancy was unexpected and an attempt was made to obtain closer agreement between the two instruments.
The average time delay value of
0.849754 ns was referenced to an
open circuit at the TDR sampling
head, meaning the connection at the
head was not terminated. The reflection from the resulting open circuit
was stored as a reference waveform.
Measurements of Sample 6 were taken with respect to this reference. To
improve the agreement between TDR
and VNA measurements, the sampling head was fitted with a 3. 5 mm
pin to 3. 5 mm socket precision adapter (“connector saver”) from a VNA
calibration kit. The adapter provides a
precise reference plane and sufficient
electrical length to establish a new reference plane well away from the sampling head’s 3. 5 mm panel connector.
To define a new reference plane,
a 3. 5 mm (pin) precision open from a
VNA calibration kit was used. The open
was connected to the sampling head
and the resulting waveform was stored
as the new reference. TDR measurements of Sample 6 were conducted as
described under Equipment and Test
Conditions. The above-mentioned
method of reference plane calibration
was applied to the primary TDR used
in this experiment as well as a second
TDR of the same manufacturer.
To ensure TDR/VNA test parity, the VNA was re-configured from
a two-port to a one-port calibration
and best-case performance testing
was repeated. DUT time delay data
was extracted from the resulting S11
reflection data. Findings indicate virtually no change in VNA instrument
uncertainty, as compared to two-port
S21 data, and a decrease measurement
uncertainty associated with connect/
disconnect DUT testing.
Figure 9 compares the 22 connect/
disconnect performance of the TDR
with that of the VNA, when using S11
reflection measurement techniques.
As with earlier testing, the VNA’s uncertainty is approximately an order of
magnitude below that of the TDR under similar measurement conditions.
The findings suggest that before
making critical production measurements with either a TDR or VNA, an
understanding of DUT and measurement system interaction is necessary.
Each has its strengths and weaknesses,
but in the hands of a properly trained
and experienced user, both are formidable tools. Data has been presented
indicating that the VNA operates with
a significantly lower level of measurement uncertainty under specific conditions. It is left to the reader to decide which best suits his or her needs
given the application requirements. n
The author extends his thanks to
Jose G. Ramirez, Industrial Statistician, and Harmon Banning, Technologist, and W.L. Gore & Associates Inc.
for its guidance and kind assistance in
the writing of this technical note.
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