Esoteric question about fixture de-embedding with the "2x thru" method - Page 1
Forum section: electronics / rf-microwave

author: mark03
author_rank: Frequent Contributor
timestamp: June 16, 2019, 06:15:42 am
content: When using a VNA to measure a DUT mounted on a circuit board or other test fixture, corrections are necessary to obtain the S-parameters of just the DUT, removing the contributions from the PCB traces, connectors, etc. Normal VNA calibration puts the reference planes at the ends of the test cables, but moving the reference planes to the DUT is tricky. If I understand correctly, the traditional solution is to place calibration standards on the same board as the DUT and do a second-tier calibration using TRL or even SOLT, although accuracy depends on the degree to which these standards are known. It's a bit of a chicken-and-egg problem.Recently there is something called "2x thru" which figures out and de-embeds the fixture based on nothing more than a single "thru" trace. (The "2x" nomenclature is confusing; I guess 1x refers to one side of the DUT, so the entire connector-to-connector thru is considered 2x.) This involves transforming into the time domain, masking the trace, and transforming back. There is some craft involved in the particulars, leading to proprietary implementations.What I want to know, is whether the time-domain masking is still necessary if you assume that the test fixture is perfectly symmetric. In other words, suppose you hand me a two-port P in a black box, and only provide the following information:A) the scattering parameters of P, andB) a guarantee that inside, P is actually composed of two identical two-ports Q cascaded back to back.Shouldn't it be straightforward to calculate the S-parameters of Q, without a bunch of black magic? And if so, couldn't you use this procedure to de-embed a test fixture, at frequencies low enough that it's reasonable to assume the connectors and connecting traces on either side of the DUT are identical?

author: rfeecs
author_rank: Frequent Contributor
timestamp: June 16, 2019, 05:33:57 pm
content: Quote from: mark03 on June 16, 2019, 06:15:42 amNo. Since P is symmetrical, S21=S12 and S11=S22. So when you measure it, you only get two pieces of information, S21 and S11.But Q is not symmetrical. S21=S12, but S11 does not equal S22. So for Q, you have three unknowns.You have 3 unknowns, but only 2 knowns. It doesn't work.If you have a good fixture, you can assume that S11 and S22 of Q are zero. Then you just de-embed using S21. This is just adjusting for the phase and loss of the fixture only. It works well at low frequencies with a good fixture.Another method is the thru-delay cal. Here you measure your thru and a longer thru line (the delay). You make the assumption that the fixture is symmetrical. This method works very well.The best by far is TRL. You have very few assumptions: your impedance is based on the impedance of your thru line only. Your reflection does not have to be well characterized, just a good reflection and the same on both ports. Your delay just has to be the same impedance as the thru with a phase shift not too close to zero or 180 degrees.

author: mark03
author_rank: Frequent Contributor
timestamp: June 16, 2019, 08:16:33 pm
content: Quote from: rfeecs on June 16, 2019, 05:33:57 pmThanks. This was the intuition I was lacking. It makes sense to me now.My interest is mainly below 1 GHz, so even simple port extension or response cal may be adequate. But I am curious what the next step up would be if one wanted to be extra picky. TRL is not practical at low frequencies. TRM?

author: rfeecs
author_rank: Frequent Contributor
timestamp: June 16, 2019, 08:44:12 pm
content: Quote from: mark03 on June 16, 2019, 08:16:33 pmQuote from: rfeecsI realized that this is not quite right. For TRL, the impedance is based on the impedance of the line, not the thru.For TRM, it's based on the impedance of the load. So it's good for low frequencies, but it's only as good as your load, so that can be tricky to make a very good load.Reference plane extension is certainly the easiest and may well be good enough for low frequencies and a good low loss fixture.To compensate for phase and loss of the fixture but not mismatch, you can measure a thru. Take the "square root" of S21 and use that as S21 and S12 of your fixture halves, with S11 and S22 set to zero.

author: mark03
author_rank: Frequent Contributor
timestamp: June 17, 2019, 04:07:09 am
content: Agilent suggest some techniques for characterizing on-board SOLT standards in AN 1287-9 (in-fixture measurements) so that is also an option. Also saves needing to calibrate switch terms separately on three-sampler test sets like mine.As an alternative to TRM for cheap PCB fixtures, though, I wonder if this could be useful:https://ws680.nist.gov/publication/get_pdf.cfm?pub_id=18424"Series-resistor calibration"The series resistor may be easier to characterize than a load?In another paper discussing the same method,https://www.researchgate.net/profile/I_Takeuchi2/publication/224204598_A_Compact_Variable-Temperature_Broadband_Series-Resistor_Calibration/links/02e7e519ad34f58b36000000/A-Compact-Variable-Temperature-Broadband-Series-Resistor-Calibration.pdf, they discuss a way to estimate the coplanar waveguide Z ahead of time and account for that in the calibration.