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Editor's note: this article was contributed by Brian Walker who works at Copper Mountain Technologies. Thanks! He provides a good description of how to accurately calibrate to a waveguide interface. Some of the discussion references Copper Mountain test gear operation, other equipment will have slightly different procedures. With the help you get from Brian you should be able to figure out how to do this on Keysight, Anritsu or Rohde and Schwarz VNAs!
TRL stands for Thru, Reflect, and Line. It is an accurate and effective method for the calibration of a Vector Network Analyzer (VNA). Most VNA users are familiar with Short, Open, Load, and Thru (SOLT) calibration for coaxial media, but many have not tried TRL. This paper provides an introduction to the technique and demonstrates how useful it can be for calibration of a measurement system employing waveguide.
What Calibration Pieces are Needed?
To perform waveguide TRL, one will need the “Reflect”, which can just be a shorting plate. The “Thru” will amount to simply connecting the two waveguide feeds directly together, a “zero length” Thru. The “Line” needs to be between 20 and 160º longer than the Thru—which is 0º for the zero length Thru—and must have excellent return loss.
Figure 1 - Two WR-229 Feeds, a Short and a Line
Figure 1 shows two coaxial to WR-229 waveguide feeds, the shorting plate, and the Line. WR-229 operates from 3.3 to 4.9 GHz. To use these for TRL calibration create a calibration kit in the VNA. It is particularly important to know the delay of the Line section, and to verify that its phase is between 20º and 160º with respect to the Thru over the band of calibration.
As an example for measuring the delay of the Line, set the frequency range on the VNA to 3 to 5 GHz and perhaps 800 points and set S21 delay measurement with a marker in the middle. Put the two waveguide feeds together to create a zero length Thru and normalize the trace to zero (Display>Memory>Normalize). Now, place the Line between the two feeds and measure the delay. It may be necessary to turn on averaging if the measurement is noisy. The Line of Figure 1 above measured 61 ps of delay. The accuracy of the delay isn’t critical, as it is only needed to resolve the ambiguity of the sign of a square root in the correction calculation.
Next, set the measurement to Phase and verify that the phase of the Line is between -20º and -160º across the band. If it is, proceed to the calibration kit table in the VNA menu and create a new entry as shown below in Figure 2.
Figure 2 - Calibration Kit Entry
Select “Define Stds” from the menu and enter the information as shown:
Figure 3 - Cal Kit Definitions
The Thru is defined with zero delay and the Line is defined by the 61 ps we measured. The short also has no delay, as it is simply attached directly to either waveguide feed.
Lastly, close the definition table and choose “Specify CLSs” to define the classes of the TRL standards.
Figure 4 - TRL Standard Classes
This tells the VNA which standard to use for which purpose. Now, select the new Cal Kit to make it active and we are ready to perform a calibration. Remove the normalization using Display>Memory>Data Math [OFF] and turn off averaging if it is engaged. Proceed to Calibration>Calibrate>2-Port TRL Cal. Place the two waveguide feeds directly together and press the 1-2 Thru/Line button. Now, put the Line between the feeds and press the 1-2 Line/Match button. Place the shorting plate over the opening of the Port-1 waveguide and press the Port-1 Reflect button. Place it over the Port-2 waveguide opening and press the Port-2 Reflect button. Press the “Apply” button and the calibration is complete.
You can check to see if the calibration is consistent by placing the shorting plate over the Port-1 waveguide and seeing that the S11 Log Magnitude reading is flat and at zero. Place the two feeds together and check for 0 dB S21 and a low reading of S11.
Additional Information
The greatest error in SOLT calibration is due to the uncertainty (worst case return loss) of the calibration load standard. This is likely to be 30 to 35 dB for an expensive kit. An Automatic Calibration Module (ACM) can achieve 47 dB. The load uncertainty sets the absolute floor for reflection measurement. If the load uncertainty is 30 dB, then reflection measurement uncertainty will be ±3.3 dB at 20 dB reflection—10 dB down—and ±1 dB at 10 dB reflection—20 dB down.
TRL calibration doesn't use a load. Instead, the floor for reflection uncertainty is set by the uncertainty or worst-case return loss of the Line standard. Since the Line is simply a machined rectangular duct, its return loss can be quite good allowing for a final reflection uncertainty floor of 50 to 60 dB! Much better than SOLT.
Multiline TRL
The 20º to 160º phase requirement for the Line sets a bandwidth limitation on the calibration. To extend the range, use multiple lines. For instance, the lowest frequency line might be utilized over a frequency range where it is 20 to 160º and a shorter line is used from 30º to 160º. The second line will be 30º at the same frequency where the first line is 160º. It is a good idea to have some overlap which is why the second line takes over where it is 30º instead of 20º. These additional lines must be added to the kit definition, perhaps named Line1, Line2, Line3 and so forth. The Class table will need to be updated for this. Line1 can be entered in Subclass 1, Line 2 in Subclass2 and so on. These lines will automatically be populated in the Calibration menu so they may be chosen during the calibration process.
Conclusion
TRL is a very accurate and easy method of calibration . We have seen here how to create a kit definition for the VNA, how to perform the calibration, and how multi-line TRL might be used to extend the range of calibration.
For more information on calibration techniques and VNA measurement uncertainties, feel free to view the webinars at www.coppermountaintech.com/rd-webinars/.