HP 8719ET Reference Manual page 275

Determining System Measurement Uncertainties
Determining Cable Stability Terms (C , C , C , C , C , C )
R1 R2 TM1 TM2 TP1 TP2
Determining Cable Stability Terms
(C , C , C , C , C , C )
R1 R2 TM1 TM2 TP1 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 10-1, Figure 10-2, and Figure 10-3 show an example that demonstrates the
concepts useful in determining cable stability. In each case, a cable (HP 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 with the free end held close to port 2 and storing the
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results in memory. Two additional S measurements are made; one with the cable flexed
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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 10-1 demonstrates the stability of the
cable when moved to its original position.
Figure 10-1 Cable Stability with Fixed Load Connected
Figure 10-1 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
(−60/20)
estimated as 10 or 0.001.
10-6