THE IMPORTANCE OF USING CALIBRATION STANDARD DEFINITIONS FOR A VECTOR NETWORK ANALYSER page 4

Table 1 Offsets with DEFAULT definitions.

Type Sex Frequency @ Upper OFFSET LOSS Limit of 0.033dB (MHz) Frequency @ Upper PHASE ERROR Limit of 0.33 deg (MHz) DELAY OFFSET ERROR at 0.3 MHz (ps)
BNC SHORT Male 159 10.6 -84.609
OPEN Male 288 10.9 -86.219
SHORT Female 110 15.3 -41.384
OPEN Female 650 22.8 -42.355
SMA SHORT Male 272 21 -26.929
OPEN Male 1780 23 -28.809
SHORT Female 709 26 -21.462
OPEN Female 1350 28 -24.035

COMPARING STANDARDS

 A large improvement over the BNC performance results obtained in Table 1 can be made by entering the the characteristics of the standards into the VNA definition table. This is because the errors in the BNC connector standard are far greater than those of a dedicated calibration kit. The home made BNC(m) standard is basically a long centre conductor in a long tube with a solid dielectric, which results in a large capacitance and also an even longer electrical length due to the resulting velocity constant caused by the solid dielectric. Whereas the comparable Agilent N 50 Ohm standard is little more than a centre contact virtually supported in mid air within a screening 'tube'. The difference between the termination Box BNC(m) OPEN and the Agilent Type N 50 Ohms Male OPEN can best be appreciated by looking at Figure 10 which shows the capacitance vs. frequency of the two standards. The delay of both standards has been included into C0 in order to give an easy visual comparison.

[Graph showing capacitance of Agilent N 50 Ohm standard compared to BNC connector standard]

STANDARD DEFINITIONS

 The Termination Box was fitted with different connector types and the characteristics determined by our measurement method (see Table 2).  It was not possible to find the ideal capacitor coefficients for any of the BNC OPEN 'standards' because at least one coefficient for each standard exceeded the VNA input range of ±10K. This meant that the coefficients had to be selected manually and were therefore subject to selection interpretation.

Table 2 USER definitions of characterised standards.

Type Sex C0 10-15 F C1 10-27 F/Hz C2 10-36 F/Hz2 C3 10-45 F/Hz3 Delay (ps) Loss (MΩ/s)
BNC SHORT Male 101.34 1300
OPEN Male 0 1500 10,000 500 100.71 1300
SHORT Female 58.079 1300
OPEN Female 6 5,000 -3,500 10,000 56.846 1300
SMA SHORT Male 43.624 1300
OPEN Male 0 -2140 3534 -688 43.3 1300
SHORT Female 38.157 1300
OPEN Female 0 -5617 -2448 347 38.526 1300

RESULTS WITH USER DEFINITIONS

 The residual OFFSET PHASE was calculated from the measured phase characteristics of the BNC(m) OPEN standard and the user values given in Table 2.  Figure 11 shows the resulting Phase errors for OPEN standards and it can be seen that the BNC(m) connector standard is now usable up to 2.0 GHz  for Phase within the previously set limit of 0.33°. The exercise was repeated for the BNC(f) OPEN connectors and then both the SMA OPEN connectors and the results are shown in Figure 11 and Table 3.

[Graph showing calculated OFFSET phase of OPENS using USER definition]

The residual OFFSET PHASE was also calculated for the BNC(m) SHORT standards.  Figure 12 shows the resulting Phase errors for SHORT standards and it can be seen that the BNC(m) connector standard is now usable up to 1.92 GHz  for Phase within the previously set limit of 0.33°. The exercise was repeated for the BNC(f) OPEN connectors and then both the SMA OPEN connectors and the results are shown in Figure 12 and Table 3.

[Graph showing calculated OFFSET phase of SHORTS using USER definition]

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