Specifications

9-6
Determining System Measurement Uncertainties
Determining Cable Stability Terms (C
R1
, C
R2
, C
TM1
, C
TM2
, C
TP1
, C
TP2
)
Determining Cable Stability Terms
(C
R1
, C
R2
, C
TM1
, C
TM2
, C
TP1
, C
TP2
)
Cable stability is dependent on the cable used and the amount of cable movement between
calibration and measurement. Values for cable reflection stability are determined by
connecting a fixed load to the free end of the cable and measuring the change in reflection
coefficient after flexing the cable through the normal range of cable movement for a
particular setup. Cable transmission stability is determined by connecting a short to the
free end of the cable and measuring the change in reflection coefficient due to changes in
cable position.
Figure 9-1, Figure 9-2, and Figure 9-3 show an example that demonstrates the concepts
useful in determining cable stability. In each case, a cable (part number 8120-4779) was
connected to port 1, with a fixed load connected to the free end. A reference trace is
obtained by measuring S
11
with the free end held close to port 2 and storing the results in
memory. Two additional S
11
measurements are made; one with the cable flexed out to its
straight position and the other with the cable positioned back to the same location as
reference trace. The flexed position demonstrates the effect of moving the cable after
calibration, the repeatability trace in Figure 9-1 demonstrates the stability of the cable
when moved to its original position.
Figure 9-1 Cable Stability with Fixed Load Connected
Figure 9-1 demonstrates the concepts useful in determining cable reflection stability. A
fixed load is connected to the free end. The DATA-MEM feature provides an indication of
the cable reflection stability. A 60-dB peak on the chart yields a reflection stability
estimated as 10
(−60/20)
or 0.001.