I would like to seek your advice.
We have been working on flexible microwave electronics and created a custom SMA end launch connectors to meet our requirements. However, in order to evaluate devices' performance, we are trying to extract s-parameters of our custom made end launches with follow de-embedding procedure, especially for our temperature-dependent measurements.
We have produced PCB (RO3003 62 mils) with Open, short, and through microstrip and CPW lines, attached our connectors, and measured in the same temperature conditions as our flexible devices.
The most important step is to know expected PCB s-parameters, and here we have a problem. Initially, we started with CST simulation results (Frequency and Time domain); it did not give good de-embedding results. We have tried Feko, but it seems simulation results are a bit strange, I guess, due to weird port constraints.
There is a number of publications stated that the Sonnet software could provide extremely accurate simulation results. We have tried this and partially succeeded. The through microstrip line with a length of 10mm showed expected simulation results from 0.2 to 6GHz and unphysical results from 7 to 15GHz (Our band of interest is 0.2-15GHz). We have assumed that unphysical results (Positive S21 or S21 sin-like behavior) come from box resonances, and we followed recommendations from the user guide. We tried to shrink box as we could, but coupling to the box walls disturb results. Free space lid, absorber layers, vias, and diagonal positioning of mstrip do not help much. Unphysical behavior is either improving a bit or shifting to higher frequencies.
1) What is the most accurate way to extract simulation results from our PCB?
2) If Its Sonnet, then how can we fight with this issue or improve simulation results? Do we miss something?
3) Any recommendations or suggestions in our procedure?
I was 10 years in the on-wafer test business before I changed in a company where we are making calibration kits on coaxial basis.
At first forget the simulation tools. They are good, but never good enougt for this aspect and they are only so good as the input is. You have a lot of material and traces with dispersive behaviour, you get couling with the surroundings, you get temperature drift and additional dispersion with any plastic and the calibration elements are never good enought.
We developed 15 years ago a concentrated calibration kit on a PCB board to calibrate robust PCB-RF Probes in GSG and GS designs up to 4 GHz (IZI-Probe PCB, works up to 20 GHz). But the quality and bandwith was never comparable with calibration cits on ceramic. And even 15 GHz on PCB are high!
What can you do now? I believe you should use a fitting coaxial calibration cit and measure the S parameters of your complete calibration structure between well defined coaxial ports. If you have only planar ports require it additional deembeding of the connection between. Than can you use these S parameters as kit for deembedding. If you measure at different temperatures do it again under these conditions.
I suspect that your substrate at 62 mils is too thick, at least at the higher frequencies. As I recall this can cause strange propagation modes and non-physical effects such as you have observed, Try a substrate thickness on the order of 20 mils. The simulation tools should predict these modes and effects, so you can try it first in CST. You may see slightly higher insertion loss with a thinner substrate.
I wrote a critical review on the calibration standard in my sophomore year. You are really using a thick substrate. Try to replace it. It should work.
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