Series through impedance measurement with Bode 100 | Working principle

[nico98]

Hi all,
currently i am working with the Bode 100 VNA to analyze impedance spectra. Since my DUT is far above 10 kOhm impedance, the user manual suggests to use the Series through measuring set up. On the internet i could find a lot of research on the port setup but it was never explained on how this setup work in detail. Why does the series through setup allow to measure high impedances whereas the oneport method doesn't? Is it because the DUT is put in parallel with a short connection which lowers the overall impedance or has it something to do with the internal reference resistors?
Cheers, Nico

[mtwieg]

Here's a document explaining in detail why certain measurement methods suit different DUT impedances:

https://www.mwrf.com/technologies/test-measurement/article/21849791/copper-mountain-technologies-make-accurate-impedance-measurements-using-a-vna

Basically with a shunt method, as the DUT impedance approaches infinity, incremental changes in impedance have no effect on the measured S11 or S21, even though the measured signals are still large. With a series connection, as the DUT impedance approaches infinity, incremental changes in impedance will still cause significant changes in S21, even though the measured signal at port 2 may be tiny. If you carry out the math, the latter case gives better accuracy in the final impedance measurement.

[nico98]

okay thanks, that helps a lot. So as far as I understand is the impedance not "too high to measure" in a oneport set up, but the variance goes up to infinity at high impedances whereas the variance for the series thru set up goes down with higher impedances. I thought the reason is a technical one but this makes sense.

[mtwieg]

As impedance goes towards infinity, the series method accuracy will also degrade (because your S21 measurement will hit the instrument's noise floor), but the overall occuracy will still be better than the shunt method.