Using Kirkby Microwave calibration kits with the VNWA.

All our coaxial calibration kits can be used with the VNWA from SDR-kits. In fact Kirkby Microwave own a VNWA, in addtion to several higher end VNAs. The VNWA supports three calibration models:

  1. Ideal calibration standards. These just assume simple theoretical text-book standards, and are not suitable for use with any professional VNA calibration kit.
  2. Simple model custom calibration standards. These allows delays of standards to be entered, but not the fringing capacitance, so again is not suitable for use with professional calibration kits.
  3. Arbitrary model custom calibration standards. This is the model that should be chosen to use Kirkby Microwave calibration kits, as the other models are too simplistic. See the image below.

Arbitrary model calibration standards used when using Kirkby Microwave calibration kits with the VNWA

This model is very flexible, but is not as easy to use as other VNAs. The calibration standards are represented as a mathematical model, which can be expressed as either S-parameters (S), an admittance (Y) or as an impedance (Z).

Setting the load in the VNWA calibration kit defintions

Setting the load when using Kirkby Microwave calibration kits with the VNWA

For the load use the impedance, and enter a value of 50. Our loads are characterised for a low reflection coefficient from DC up the maximum frequency of the calibration kit. Do not use the DC resistance, as SDR-kits suggest. Despiting having a calibrated HP 3457A 6.5 digit multimeter, with support for 4-wire resistance measurements, we do not measure the DC resistance, and suggest this is not done, even if you have a suitable bench multimeter. This is because the DC resistance tells you nothing about how the device acts at RF frequencies. Set either S=0 or Z=50 for the load. Just set the delay to 0, as that's irrelevant. Press carriage return to compile the data entered.

Setting the short in the VNWA calibration kit defintions

Set either Z=0 or S=-1 for the short. Set the delay to the NEGATIVE value of the of the standard in the Kirkby Microwave kit. Typical values that would need setting would be from -75 ps to 0 ps, depending on the model of kit, and the gender of the calibration standard.

Setting the short when using Kirkby Microwave calibration kits with the VNWA

Setting the open in the VNWA calibration kit defintions

The open standard is the most complicated to define. It is possible to compute the S-parameters, impedance or admittance of the open standard, but using the admittance is easiest. The open standard has a fringing capacitance C, which is frequency dependant. The admittance of a capacitor is:

Y=j*ωC where j=sqrt(-1) and ω = 2*πf

The fringing capacitance of the open calibration standards follows the method used by almost all VNA manufacturers - a 3rd order polynomial.

Cfringing=C0x10-15 + C1x10-27f + C2x10-36f2+C3x10-45f3 farads, where f is in Hz.

If a kit had an offset delay of 50 ps, and the fringing coefficients were C0=60, C1=70, C2=80 and C3=90, the admittance would be written in the VNWA as below, where the VNWA uses w insted of ω

Y=j*w*(60e-15 + f*70e-27 + f*f*80e-36 + f*f*f*90e-45)

Kirkby Microwave open defined for the VNWA

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