FAQs from Rohde & Schwarz

ZVL time domain measurement, impedance over cable length


I want to measure the impedance over the compete cable lenght with my ZVL.

After my first try it is possible to switch to the time domain measurement showing impedance, but I'm not able to change the display to the desired lenght.

Also I do not see the correct course of impedance. The ZVL just displays the peaks e.g. at the coupling joints. On the open end I see a big peak, but then again 50 ohms - this looks strange at first view.

How can I improve my settings?


These are the steps to perform the desired measurement (Ref. Firmware V. 3.32)

- Disconnect all ports

- Press [MODE]

Choose "Network Analyzer"

- Press [PRESET]

--> Now the complete frequency range of your analyzer is automatically choosen. This is very important for the resolution in TD measurements

- [MEAS] - (Impedance) - (Z<-S22)

- It is very important to use S22, because the velocity factor correction will only be applied to Port 2.

- [Trace] - (More) - (Time Domain Transform) - (Time Domain)

- At this point only a time range of 5ns (-1ns to 4ns) is displayed. You can see a line at 50 Ohms except at the reference point.

- To improve accuracy also increase the number of Points: [SWEEP] - (Number of points) - Set it to the maximum of 4001 points

- [TRACE] - (More) - (Time Domain Transform) - (Stimulus Transform) allows you to choose (Distance) measurement and also to specify the desired length with (Start) and (Stop). For the example let's choose start at -10mm and stop at 3m.

- [TRACE] - (More) - (Time Domain Transform) - (Define Transform) - choose "Low Pass Step" and change the "Low Pass Settings" to "Stop Frequency and Number of Points".

- Now it's also important to choose the right "DC Value". It will be "0 U" for a 50 Ohm termination at the end and "1 U" for and open ended cable.

- Finish that setup by pressing "Close" twice - the Display should now show the right curve

- At the end you should not forget to enter the velocity factor of the cable, so that the right length will be displayed.

- [MEAS] - (More) - (Define Distance To Fault) - Cable Type) - and choose an appropriate cable type.

- If you can not find the cable, you can add a new type when clicking on "Available Cable Types".

- The device is not calibrated yet. Perform a One-Port P2 calibration and connect the cable afterwards.

- Now you can perform the final measurement.

This example shows the impedance through a stretch of 2 cables consisting of 2m RG58 (50 Ohm) and another cable (RG59 / 75 Ohm) afterwards.

ZVL time domain measurement, impedance over cable length