Introduction: What is Calibration?
Calibration is critical to making good VNA S-parameter measurements. While the VNA is a highly linear receiver and has enough spectral purity in its sources to make good measurements, there are several imperfections that limit measurements done without calibration.
Calibration is a tool for correcting for these imperfections, as well as other defects. There are an enormous number of possible calibration algorithms and many of them are implemented within VNAs. The choice between them is largely determined by the media the engineer is working in, the calibration standards available and the desired accuracy/ effort trade off.
Why do we need to calibrate? Measurements performed by an uncalibrated VNA are measurements of everything between its ports, usually including adapters and cables, and sometimes including amplifiers, PCB traces, probes or other components, as well as the Device Under Test (DUT). By calibrating at the interface of the DUT, we mathematically remove the losses and delays of the fixture from our measurements, so we are left with the measurement of the DUT alone. And that is why calibration is important.
Types of Calibration
Here we concentrate on 2- and 1-ports.
- Full 2-Port - This is the most commonly used and most complete calibration involving two ports. All four Sparameters (S11, S12, S21, and S22) are fully corrected.
- Full 1-Port - In this case, a single reflection parameter is fully corrected (either S11 or S22). Both ports can be covered but only reflection measurements will be corrected. This calibration type is useful for reflection only measurements, including the possibility of doing two reflection-only measurements at the same time.
Types of Measurement Errors
There are three different types of measurement errors when using a VNA – drift, random and systematic errors.
- Drift Errors are due to the instrument or test-system performance changing after a calibration has been done. Drift is primarily caused by temperature variation and it can be removed by recalibration. The time frame over which a calibration remains accurate is dependent on the rate of drift that the test system undergoes in the test environment. A stable ambient temperature usually minimizes the rate of drift significantly. Allowing equipment to warm up and stabilize prior to calibration and properly ventilating equipment helps reduce drift errors.
- Random Errors are unpredictable since they vary with time in a random fashion. Therefore, they cannot be removed by calibration. The main contributors to random error are instrument
noise such as, source phase noise, sampler noise, and IF noise. The accurate source and phase-locked receiver of the network analyzer greatly minimizes these random errors. There are also external contributors to random errors such as switching power supplies, EMI, etc.
- Systematic errors are related to signal leakage, signal reflections, and frequency response of the test system. There are six types of systematic errors – directivity, reflection tracking, transmission tracking, source match, load match and isolation.
General Best Practices
- It is always a good practice to warm up the VNA before using it to get accurate measurements.
- Second important thing is to check if the cables are in a good condition. A quick way to do this is to connect the cable end to Short, Open and Load and see the corresponding S11 graph in Log Magnitude.
- Both the open and short connections must yield a flat line very close to 0dB. The load connection must give you a very low return loss (around -40dB).
- Always make sure to use a torque wrench while making the connections.
- Also make sure that the VNA is in an ESD (Electrostatic Discharge) safe environment and the operator is wearing an ESD wrist wrap.
- With these pre-checks performed, you can now go ahead and make the measurements
Pre-lab
The information for this pre-lab can be found in Chapter 5 of the VNA’s user manual (You can find the operating manual under C:\VNA\S2VNA\Doc)
- Identify the 6 systematic error categories as well as the S-parameter it affects (S11 or S21).
- What is the difference between a one-port and a two-port calibration? What is the purpose of each of these? That is, why would you perform one instead of the other or should one always be used for the most accurate measurements?
- In your own words, define the term “calibration plane”.
Objectives
- To learn how and why to calibrate a VNA.
- To understand the calibration plane and its relationship to VNA measurements.
Lab Procedure
Section 1 – Changing Measurements
- Turn the VNA on if it is not already and launch the S2VNA software.
- Connect cables to ports 1 and 2 of the VNA and then connect an attenuator between two SMA cables.
- To add traces, go to “Display” on the side bar, then click on Num of Traces and change it to 4
- To allocate these traces, go to Display option on the sidebar menu and click on Allocate Traces and then select the quadrangle arrangement at the bottom of the drop window.
- Measure the attenuator without calibration
- Measure the S parameters (S11, S12, S21, S22 of the device in log magnitude and take a screenshot of the results for reference.
Section 2 – Calibrated vs. Uncalibrated Measurements
- Before we start with our calibration procedure, turn the VNA on and launch the S2VNA software.
- It is always a good practice to make the stimulus setting changes before calibrating the unit. To do this, click on Stimulus on the side bar menu and set the start and stop frequency to 1MHz and 8GHz respectively. As a starter, let the points be 201 and the IF Bandwidth be 10 KHz.
- The calibration kit definition is where the instrument is told the actual, known characteristics of the calibration standards. Each calibration kit has its own known characteristics. In general, you will be using SMA cables.
Cal kit. The CalKit number appears on the tool; make sure you put the right kit in the drop-window menu. Note: our model differs from the example
- Go to Calibration on the sidebar menu and click on Cal kit. This must have the Cal kit number displayed (Our calibration kit is of N-type, N911/912 and not SMA 2.1)
(This portion is for knowledge base only - we do have the calibration unit in Cal kit drop window). If you cannot find the right Cal kit name, you will have to add the definition file first. To do so follow the steps below:
- Go to Calibration > Cal Kit. You will see a window pop on the bottom half of your screen.
- Click on a blank space under label and type SMA 2.1 and then click on Load From File. From your sidebar. Once you do this, you will have a pop up to choose a file from your computer.
- Now find a file with the name SMA 2_1 CMT.ckd and then click on open to load the file into the software.
- You will now see SMA 2.1 appear under the Cal kit option.
The Calibration Procedure
- Next, select the option for Full 2-port Calibration (Calibration -> Calibrate -> Full 2-port Cal)
- To perform the calibration, connect the “Open” standard to the end of the cable on Port 1.
- Click the “Open” button on the screen. It should say “calibration in progress…” followed shortly by a checkmark on the left part of the button, as shown in the Figure below. (Our calibration kit is of N-type, N911/912 and not SMA 2.1)
- Repeat (a) and (b) with the Short and Load standards with the cable attached to Port 1.
- Similarly, repeat (a), (b) and (c) for Port 2.
- Now connect the “Thru” standard between the two ports through the cable (the short solid short connector with male connectors on both sides). When it’s attached, click the “Port1-2 Thru” button and then click on the Subclass 2 UnThru option as shown in the figure above. Note: our calibration kit is a cross, N-type and its number is written on the unit
- When there is a check mark next to each of the seven standard names and click “Apply.” The program will process the measurements, computing differences between the measured characteristics and the known correct characteristics provided in the calibration kit definition earlier. These differences are due to non-idealities of the VNA itself and losses in the cables and adaptors used. The error correction will automatically be enabled.
(Our calibration kit is of N-type, N911/912 and not SMA 2.1)
- Save the calibration result to a calibration file on your computer (optional). To do this go to Save/Recall on the sidebar and ensure that the Save Type is State & Cal. Note: This file can be sent to your lab partner if you wish. They will just need to put it in the correct program folder. Any time from now on that you are using the VNA, the program will automatically load this calibration file. However, to do so, it is very important to use the same devices, cables that were previously used and to perform in the same environment. Calibrating the VNA every time before using it is highly advised to obtain accurate measurements.
- On the initial calibration interface (directly off the main menu), Correction should now be ON. If it says OFF, click once and it should now be activated. This will apply the current calibration settings to the measurements being made.
- Connect the attenuator used earlier and make the same measurements again. You must be able to see a difference in your S11 and S21 measurements in comparison to the one made without calibration.
- Screenshot this image and include it in the lab report. Also make a note of any differences you see between the two results.
Section 3 - The Calibration Plane
The term “calibration plane” refers to the interface at which calibration has been performed. Measurements at any other interface will include effects of not only the Device Under Test (DUT), but also the non-idealities of cables, connectors, etc. between the calibration plane and the DUT.
- Disconnect the device and remove the cable from Port 1.
- Reconnect the device, attaching it directly to Port 1 and using the cable to connect it to Port 2.
- Measure the device once more in log magnitude and take a screenshot for your lab report. Note any differences between this result and the one directly after calibration. Be sure to discuss why there are variations in terms of a “calibration plane.”
- Explain how you could calibrate the VNA to obtain results more closely matching those from the previous measurement (you don’t need to repeat the calibration; just describe how you would do so).
- Use an "unknown" termination – what do you see?
"Leading" Lab Report
- Explain what is calibration and the importance of calibration.
- Report a screenshot of the measurement made without calibration.
- Include a screenshot of the measurement made after calibration.
- Which type of errors are corrected in the process of calibration. How?
- What was the difference between calibrated vs uncalibrated measurements?
- What difference could you notice in your measurements made in Section 3?
- Use MultiSim or AWR to analyze the three segments of your load (two SMA cables and the attenuator).
- Assess the results with and without calibration. Now assess the attenuation with only the two cables connected together without the attenuator. What are the conclusions from the simulations?
- Compare the simulations to an analytical solution for SMA/Coaxial cables