Introduction: Why do we need to measure cables?
Cable measurements are required to verify and troubleshoot the electrical performance of RF and microwave transmission systems. Measurements are made on coaxial cables or waveguide systems. During the installation, operation and maintenance of an RF or microwave system, the connecting cables may become damaged or show reduced performance over time. Therefore, these cables need to be tested. The cable plays a crucial role in overall measurement of the system. As the signal travels through the transmission path, some of the energy will be dissipated in the cable and the components. A Cable Loss measurement is usually made at the installation phase to ensure that the cable loss is within manufacturer’s specification.
The measurement can be made with a portable vector/scalar network analyzer or a power meter. Cable Loss can be measured using the Return Loss measurement available in the cable and antenna analyzer. By placing a short at the end of the cable, the signal is reflected, and the energy lost in the cable can be computed.
Cable Loss Effect on System Return Loss
The insertion loss of the cable needs to be taken into consideration when making system return loss measurements. Even though this is something system designers take into consideration when setting up the specifications of the site, it is important to be aware of the effects the insertion loss and cable return loss can have on the overall system return loss. A very good system return loss may not necessarily be a result of an excellent antenna; it could be a faulty cable with too much insertion loss and an antenna out of specification. This would result in a larger than expected signal drop and once the signal reaches the antenna, a great portion of the signal is now reflected since the match is worse than expected. The result is that the transmitted signal is lower than needed and the overall coverage area is now affected. In other words, if your system return loss is too good, it is not always a good thing. A good cable is expected to have high return loss (S11) and a low insertion loss (S21).
General Best Practices
Before starting this lab, there are few practices one should keep in mind. They are as follows:
- 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 0 dB. The load connection must give you a very low return loss (around -40 dB).
- 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 prechecks performed, you can now go ahead and make the measurements.
General Full 2-Port Calibration Procedure for CMT804U (Link to this complete calibration procedure video - https://coppermountaintech.wistia.com/medias/90gsd3yb3x)
- Before you begin the actual calibration procedure, it is always a good practice to have the stimulus settings ready and then start the calibration.
- To begin the stimulus settings, click on the stimulus option on the sidebar and set the start frequency to 1MHz and stop frequency to 8GHz.
- Next, set the number of points to 401. And leave the IF bandwidth unchanged (i.e. 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. You will be using N911/912 Cal kit.
- To ensure you are using the right calibration kit, follow the steps below: Go to Calibration on the sidebar menu and click on Cal kit. This must have the Cal kit number displayed. Then, select the right Cal Kit (Note: The Kit number is indicated on the N-Type cross kit N911/912))
- For a full two port calibration, follow the steps below:
- Correction option under calibration should now be ON as shown in the Figure 8. If it says OFF, click once and it should now be activated. This will apply the calibration settings to the measurements being made.
Pre-Lab
- Why is measuring cable loss so important?
- What does cable characterization give us?
- What happens if you use a cable that is in a bad condition or the one that is lossy?
Materials
- Copper Mountain Technologies VNA (804U)
- University Kit Hardware (N-SMA cables, Band Pass Filter, Wi-Fi Antenna, Torque Wrenches (N and SMA), 2 SMA-SMA adapters, Calibration Kit N911/912)
Objective
To learn to characterize a cable using a VNA. Lab Procedure
Lab Procedure
- Connect the USB cable of the VNA to your computer. Turn the VNA ON.
- Launch the S2VNA software.
- Connect the N to SMA cables to each one of the ports of the VNA.
- Make sure you have the stimulus settings ready i.e. set the start, stop frequencies and the number of points before you calibrate. (1MHz to 8GHz for CMT 804U).
- Now, go ahead and make a Full 2-Port calibration.
- Once you are done with the calibration, go ahead and connect the cable to be measured (DUT) to the VNA cables. The setup should look like the picture given below.
- Change the number of traces to 4. To do so, click on Display > Num of Traces and enter 4.
- Allocate the traces by following Display > Allocate Traces and click on the 8th option.
- Make sure that the parameters in the four traces are S11, S21, S12 and S22.
- Let the formats of the various S parameters be as follows: S11 and S21 (Log Magnitude), S12 Delay and S22 [Smith (R+jX)].
- The cables must yield a return loss around -20 dB and the insertion loss around -1.7 dB. Delay must be close to 2.8 ns and the impedance approximately 50 Ω.
- Disconnect one end of the cable(s) and take a screen shot of the frequency and temporal signals. Explain what is that you see.
- Disconnect one end of your cable(s) and connect a short to it. Take a screen shot of the frequency and temporal signals. Explain what is that you see.
Note: The value of return loss can be read using a marker search and setting it to the maximum. Similarly, the value of insertion loss can be read using a marker search set to minimum.
Lab Report
- What is the value of the return loss (S11) you measured?
- What is the value of the insertion loss (S21) you measured?
- Include a screenshot of your measurements with all four S parameters in it.
- What was the value of the delay and impedance in your measurement?
- Using HFFS, MultiSim, AWR to simulate the transmission through the cables in frequency and temporal domains. Also simulate the cables when open and short. You may also use an analytical expression for SMA/coaxial cables.