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I also looked at 221. I note that ifftshift is used correctly in the latest version but I had also spotted it was wrong in an earlier version. I had previously checked that the centre and stop start were calculated properly and in my case they are but I am using start as negative and stop as positive and so the minus sign worked just fine.
Hi Julien,
I am still continuing to work on the time gating of S parameters and uncertainty. I recently had cause to look at the Sckit-rf repository and although I am not very familiar with the workings of
Github I could see that there is a version of the tome gating software based on the FTT rather than the convolution. Whilst these are mathematically equivalent from my work I know that you need to window the frequency domain data to avoid edge effects before transforming to the time domain. This is well known in signal processing but can be awkward to get your head around.
Below I have plotted some data from a mismatched line time gated with the current convolution method and also with the FFT method on Github.
I am happy to help progress this if needed but I am not very familiar with Github.
Regards
Roger
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Dear Julien,
It was a fluke that I picked up on this.
I do have some code that I was using to develop the uncertainty work that I am doing for ESA. NPL in the UK measured a mismatched line and provided the data in an excel spreadsheet. I read it in and was time gating it in sckit-rf just to show them that it worked as in the PNA. PTB national metrology lab in Germany have a method for propagating uncertainty without differentiation and it was this that we are trying to evaluate.
The figure I provided shows time gated data using the current convolution method and the new FFT method given by your link. I know you have to window the frequency domain data to avoid the roll off at the edge if you do this in the time domain. This is something I have played around with in the past. Of course you could make it a requirements of the user to window the data before time gating. Not so neat but possibly acceptable.
Let me know if I can help further.
Regards
Roger
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Dear Vincent,
Thanks for getting back to me so promptly. Are you going to the European Microwave conference in Milan next week if so perhaps we can meet up and discuss this in person.
I have done a lot of testing on the Convolution method both with simulated and measured data and for what we are using it for it gives very good results. We do use it with Beta = 10 following a sensitivity analysis on all the gating parameters. Our use may be unusual in that we are measuring in a quasi-optic system and using time gating to remove system artefacts e.g. standing waves etc. I do not see these boundary effects in our work and I have only ever used the default reflect method.
I will read through the whole discussion in slower time to properly absorb the decision making and understand everyone’s position.
I have included the results for the mismatched line taken with the PNA for your convenience below this email. This is obviously not quasi-optic data but something we are working on with NPL and PTB. Metrology labs in UK and Germany for propagating uncertainty.
A few years ago I did some work comparing the FFT and convolution methods to support the uncertainty work and followed the methodology in the attached presentation. I still have the test harness written in python and I can probably share it with you. I would need to seek permission from the European Space Agency. In this case I was using simulated data for silica with added noise. The silica is 15mm thick with permittivity (3.803 -0.001) and in the figure below you see the convolution and FFT overlapping but I could only get this result if I windowed in the frequency domain otherwise the FFT will roll off at the edges. The no noise data is a convolution.
I
I can work with you to resolve this but let me review the discussion and then I can consider what I can do. What is your timescale?
Mismatched line data
b = 3.50 mm, a = 1.52 mm and c = 2.31 mm. This produces a 50 ohm characteristic impedance at both inputs to the line, and a central section where the characteristic impedance is 25 ohms.
Fig 4. S11 time-domain response
Fig 8. Frequency-domain response of the mismatch at l1 (in the absence of the mismatch at l2)
Figure 8 should be compared with the plot I previously provided.
Regards
Roger
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Dear Roger,
I’m following your discussions already for a while, and I’m highly interested in your work. I just wanted to let you know that I’m also doing measuring on a quasi-optic system and experienced the roll off at the band edges (using the FFT time gate). Do you have a reference and maybe an example on the uncertainty propagation? So far, I was not really able to do a proper uncertainty analysis on the time gating part of my measurements. I have only included uncertainties from sample characterization as they were pretty dominant so far. But they can definitely not explain the “oscillations” in frequency domain which are caused by the time gate (as far as I understood).
If you are interested I can share an example dataset. For instance of fused silica like in your simulated case. By the way I’m working in the range from 220 GHz to 330 GHz, basically only limited by the availability of the hardware.
Best,
Max Kellermeier
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Dear Max,
I am currently writing a paper on the propagation of uncertainty through the time gating process. The paper is being prepared with colleagues at PTB and NPL as I mentioned earlier. One of the data sets we are using is the mismatched line. We will also present data for silica.
I would be happy to look at some of your data and see if I can help in your understanding. In our case we started time gating calibrated data to remove instrument artefacts. This was an essential step in cleaning it up before extracting permittivity. If you can send me your calibrated data for silica as a touchstone file I can easily put it through our process and explain what I have done. This will not include propagation of uncertainty as this is still work in progress.
Could I ask where you are doing your research?
Regards
Roger
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Hi Vince,
I understand the difficulty in attending conferences and I have to limit myself to one a year! Where are you based just in case I am passing that way.
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