Great topic! I’m learning about microwaves and antenna’s right now so this couldn’t be timed better!!! Would love to see more in-depth videos on this topic :)
As the developer of the LibreVNA (and user of Altium Designer) I certainly appreciate it being used as a demo device here. However, I can't help but be a little disappointed in the execution: 1. A very important part of any VNA measurement is the calibration. The calibration standards (and their assumed limits) are briefly mentioned in the video but the LibreVNA is *not* actually calibrated during the live measurement (as indicated by the red background of "calibration" in the upper right toolbar). This immediately makes every measurement essentially worthless. Yes, the ripples still give an indication about the line length but the measurement is dominated by errors from the VNA frontend (which would have been calibrated out if a SOLT calibration had been performed) 2. The LibreVNA supports conversion of the S-parameters to time domain data (TDR mode in a VNA). This would have allowed to plot the trace impedance live on screen (showing trace impedance in Ohms vs. distance). Of course this transformation only yields valid results if the VNA is calibrated. I get that this is an instructional video and should show the math behind that as well but it would have been a nice addition. Although I am certainly a little bit biased when it comes to the LibreVNA, I can't really compare it to professional VNAs (neither in performance nor in price). But I certainly feel that it has been a little bit underutilized in this example.
It was calibrated prior to usage. Not sure why the box is red but it was calibrated live with the provided standards and not just settings loaded from a file. In fact in a different video I talked more about calibration and IIRC showed it live.
Thanks for the fast reply :) It may have been calibrated previously but the fact that the box is red indicates that the calibration is not active during the measurement. There is a checkbox to the right of it and that has to be checked, otherwise the calibration data will not be used. I admit that such a user error can easily be caused by my incomplete or unclear documentation. If that is the case, feel free to reach out if there are any questions regarding the usage of the LibreVNA. I am happy to help or add further explanations to the user manual
Hello i have a question concerning your video. The primary question was how to measure the trace impedance. But i could not find any concrete formula how to do this using S21 and S11 in the video. Could you please provide the exact formula. Thanks
It requires two formulas. S11 is equal to the reflection coefficient from the input impedance as seen at port 1. So you first take S11 and convert it to a decimal number; then you use that to determine the input impedance. Once you have the input impedance, you can calculate the line's characteristic impedance.
At lower frequencies it is pretty good and I've used it in some circuits just as a sanity check against a design. I'm suspicious of the higher frequency range and whether that reading is really accurate. I have never qualified it and I do not know anyone that has qualified it. But it is inexpensive compared to a higher end VNA so it's worth getting if you work at lower frequencies.
Hi Zach, Thanks for answering my question.
Great topic! I’m learning about microwaves and antenna’s right now so this couldn’t be timed better!!!
Would love to see more in-depth videos on this topic :)
As the developer of the LibreVNA (and user of Altium Designer) I certainly appreciate it being used as a demo device here. However, I can't help but be a little disappointed in the execution:
1. A very important part of any VNA measurement is the calibration. The calibration standards (and their assumed limits) are briefly mentioned in the video but the LibreVNA is *not* actually calibrated during the live measurement (as indicated by the red background of "calibration" in the upper right toolbar). This immediately makes every measurement essentially worthless. Yes, the ripples still give an indication about the line length but the measurement is dominated by errors from the VNA frontend (which would have been calibrated out if a SOLT calibration had been performed)
2. The LibreVNA supports conversion of the S-parameters to time domain data (TDR mode in a VNA). This would have allowed to plot the trace impedance live on screen (showing trace impedance in Ohms vs. distance). Of course this transformation only yields valid results if the VNA is calibrated. I get that this is an instructional video and should show the math behind that as well but it would have been a nice addition.
Although I am certainly a little bit biased when it comes to the LibreVNA, I can't really compare it to professional VNAs (neither in performance nor in price). But I certainly feel that it has been a little bit underutilized in this example.
It was calibrated prior to usage. Not sure why the box is red but it was calibrated live with the provided standards and not just settings loaded from a file. In fact in a different video I talked more about calibration and IIRC showed it live.
Thanks for the fast reply :) It may have been calibrated previously but the fact that the box is red indicates that the calibration is not active during the measurement. There is a checkbox to the right of it and that has to be checked, otherwise the calibration data will not be used. I admit that such a user error can easily be caused by my incomplete or unclear documentation. If that is the case, feel free to reach out if there are any questions regarding the usage of the LibreVNA. I am happy to help or add further explanations to the user manual
Hello i have a question concerning your video. The primary question was how to measure the trace impedance. But i could not find any
concrete formula how to do this using S21 and S11 in the video. Could you please provide the exact formula. Thanks
It requires two formulas. S11 is equal to the reflection coefficient from the input impedance as seen at port 1. So you first take S11 and convert it to a decimal number; then you use that to determine the input impedance. Once you have the input impedance, you can calculate the line's characteristic impedance.
My Libre VNA interface is different from yours, S11 and S22 are in polar plot. is there any method switch to magnitude vs frequency?
How to measure circuit impedance with the help of a librevna and calculate it with something else.
I know, but that's how VNA's work!
LibreVNA is a toy ? Serious, 3Ghz @ 100 db .. I will take it !
At lower frequencies it is pretty good and I've used it in some circuits just as a sanity check against a design. I'm suspicious of the higher frequency range and whether that reading is really accurate. I have never qualified it and I do not know anyone that has qualified it. But it is inexpensive compared to a higher end VNA so it's worth getting if you work at lower frequencies.