Verifying the performance after calibration

After using one of our kits to calibrate your VNA, it is sensible to check the VNA is giving good results. A failure to give good results indicates either a fault in the VNA or an error in uploading the coefficients properly. Unlike calibration kits from other manufacturers, all our coaxial kits include a fixed attenuator with measured data on the attenuation. The data will be found on a USB stick included with your kit in the directory "Measured_data_of_attenuator". If you get similar results to what's shown on the USB stick, there is unlikely to be any serious problems. You should get good agreement on S21 and S12, but do not expect good agreement on S11 or S22, since the return loss of the attenuator is high, so the measurement uncertainty is high.

Note the aim of the verification attenuator is not to prove your VNA is within specification - if you need to know that, send it back to the manufacturer, and not a third party lab. But the attenuator will give you a good idea if there are any serious problems.

Procedure to use the attenuator to verify there are no serious problems on transmission (S21 and S12) measurements.

This procedure is applicable to all Kirkby Microwave coaxial calibration and verification kits (i.e. SMA and N, but not waveguide).

The data measured by Kirkby Microwave will be placed in a .s2p Touchstone file. This will have the name attenuator-xxxx.s2p, where xxxx is the serial number of the calibration and verification kit. It will have the measured data on all 4 S-parameters. PDF plots of the data are also provided for the convenience of the user, although it is easy to plot them yourself, once you understand the .s2p Touchstone format. (An understanding of Touchstone files is essential if you intend making any serious use of a VNA.) You can plot the data using OpenOffice, Microsoft's Excel, or any other software capable of producing graphs. Kirkby Microwave use Gnuplot since it allows one to create files without user interaction.

As stated earlier, measuremments on S11 and S22 are subject to very high uncertainty.

One should perform the following steps to ensure the data you measure matches that measured by Kirkby Microwave:

  1. Perform a full 2-port calibration of the VNA, using the calibration and verification kit.
  2. Measure S21 and S12 of the attenuator, and save the data into a Touchstone .s2p file.
  3. Import the Touchstone file into Excel, or other spreadsheet. Use a space as the sepparator - not a tab or comma which the spreadsheet may expect.
  4. Remove the comments (lines starting with !) and the option line (starting with a #).
  5. Plot:
  6. Compare the plots of S12 and S21, to ensure that the S12 and S21 data are similar to those provided by Kirkby Microwave.

verification plot

If you measure S11 or S22 of the attenuator, and compare the data to that supplied, you will find poor agreement. This is because the attenuator is good quality and reflects very little power, especially at low frequencies. Therefore the measurement uncertainty of S11 and S22 will be high. Also, movement of cables is likely to cause changes in magnitude and phase far in excess of those of the attenuator itself. Because of the problems in using S11 and S22 data of the attenuator, a revised set of data was provided by Kirkby Microwave on some kits shipped after August 20th 2016.

Verification of reflection measurements on SMA kits shipped after 20th August 2016.

As stated above, the high return loss of the attenuator means measurements of S11 and S22 of the attenuator are subject to very high measurement uncertainty. In August 2016 we developed a new reflection standard, allowing measurements of reflection without excessive uncertainty. On SMA kits we measure S22 (at the male port) while the female port is left open, as shown below

An excellent method of verifying the VNAs ability to measure reflection data

This is not strictly a measurement of S22, since port 1 is not terminated in 50 Ω. However, this makes a very useful test in reflection mode. (It is futile trying to measure at the female port on SMA connectors while the male port is left open, as the movement of the nut will make it impossible to obtain stable measurements.)

The USB stick will contain 4 files for these measurements.

  1. attenuator-xxxx-S22-port-1-open.s1p - magnitude and phase data in a .s1p Touchstones file.
  2. attenuator-xxxx-S22-with-female-port-unterminated.pdf - magnitude and phase data in a PDF file.
  3. attenuator-xxxx-S22-magnitude-with-female-port-unterminated.pdf - magnitude data only in a PDF file
  4. attenuator-xxxx-S22-phase-with-female-port-unterminated.pdf - phase data only in a PDF file.

In each case, xxxx will be replaced by the serial number of the kit. The graphs will look simialar to this.

reflection measurement, with low uncertainty

These measurements at one port, with the other port left open, are made without any cables in the system, so the attenuator connects to the VNA only via adapters, which are electrically more stable than any cables. For measurements of transmission (S21 and S12), it is impossible to avoid using cables, although for best accuracy, only one cable could be used.

One should perform the following steps to ensure the data you measure matches that measured by Kirkby Microwave:

  1. Perform a 1-port calibration of the VNA using a female SMA or 3.5 mm connector that's connected directly to the VNA, or via adapters. Do not use cables.
  2. Measure S22 at the male port of the attenuator with the female port (#1) unterminated, and save the data into a Touchstone .s1p file.
  3. Import the Touchstone file into Excel, or other spreadsheet. Use a space as the sepparator - not a tab or comma which the spreadsheet may expect.
  4. Remove the comments (lines starting with !) and the option line (starting with a #).
  5. Plot:
  6. Compare the plots of your S22 measurements, with the female port open, are similar to those provided by Kirkby Microwave on the USB stick.

Verification of reflection measurements on N kits shipped after 20th August 2016.

For N kits shipped after 20th August 2016 we provide measuremenet at both the male and female ports while the other port is open. This is possible on N, but not on SMA, due to the fact movement of the nut on an N connector causes very little change in reflection at the female port, whereas it makes a massive difference on the SMA connector.

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