Electronics > RF, Microwave, Ham Radio

VNA One Port Fixture De-Embedding Experiment

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Nitrousoxide:
Hi All,

I'm currently conducting an experiment and would like some advice/opinions/direction from any gurus out there.

I need to de-embed a complicated fixture out of a 1 port measurement.

I'm effectively measuring a section of a custom-made ASIC to characterize the small-signal AC impedance. The chip sits on a test PCB (designed by another individual) which allows for the rerouting of signals through jumpers. The problem is that the effective multipath gives me ridiculous parasitics, even with the jumpers disconnected. I could estimate the equivalent electrical model by looking into the port- but I'm not too sure what the best way to go about that.

Additionally, the traces on the PCB are not 50 ohms (to my understanding). Since they're not 50 ohms, I've gathered I can't do a simple port extension where I just assume there's some extra transmission line present.

Calibrating up to the chip footprint isn't a possibility as I don't have a matched load of that form factor, and the analyzer I have doesn't support unknown load calibration (even if it did, I don't have anything with acceptable precision).

I've done some reading and apparently, you can measure open and short conditions to calibrate out the test fixture. The test chip sits in an elastomer socket, so it would only be possible to make open measurements, and not short. I assume this isn't sufficient?

My really silly (maybe not so) approach:
Can't I just calibrate so that the plane is at the end of my cable, connect it to my test PCB, and take "reference" S_11 measurements. After inserting my chip I can then take the S_11 measurement again, and then simply divide the measurement by the "reference". (i.e. abs(S_11/S_11,ref)).

I don't want to do a full EM simulation and parasitic extraction. It is not worth the time. I have spun a new PCB with a significantly simplified measurement chain, however, that will still need correction so the above applies.

For some extra context, I'm using a R&S ZNL. I've tried all the built-in de-embedding features but I'm not too sure if they save their offsets to exported touchstone files.

Any tips/tricks/advice (or even pointing me to some good resources, I've read most of the app notes out there) would be appreciated.

Thanks!

EDIT:
If I wanted to go down the route of finding the equivalent lumped model, I would have to estimate it somehow. Would TDR help me out? Or would it be best done in the frequency domain (as implied above).

E Kafeman:
Semiautomatic port forwarding in time and attenuation can be done in AnTune for open and/or short.
AnTune is a Windows/Labview software.
Do a regular VNA calibration, preferable with a short coaxial cable added during calibration that is around same length as the unknown transmission line, if it is a longer transmission line, for best precision if only open or short can be calibrated for.
It is then relative easy to find transmission line impedance or deembed it, setting up any complex impedance curve as reference and much more in AnTune.
VNA need to be connected to PC and AnTune use NI-VISA to communicate.
From AnTune is it easy to store and reread S11 Touchstonbe files with or without calibration added.
It is fully functional free to try.
If measuring active TX, do not overload VNA, and use AnTune function for sliding window to filter spurious.

Here is auto port forwarding time and attenuation performed for a transmission line:
https://youtu.be/RyMFun_KhAc?t=301

Nitrousoxide:

--- Quote from: E Kafeman on May 06, 2021, 12:13:56 am ---Do a regular VNA calibration, preferable with a short coaxial cable added during calibration that is around same length as the unknown transmission line, if it is a longer transmission line, for best precision if only open or short can be calibrated for.

--- End quote ---

I can calibrate out the coax cable up to the SMA connector that mates with the test PCB. That's the problem, the length of the "unknown" transmission line is.... unknown! And an even bigger problem, it may not even be a 50 ohm line, and it certainly has multiple parallel branches


--- Quote from: E Kafeman on May 06, 2021, 12:13:56 am ---It is then relative easy to find transmission line impedance or deembed it, setting up any complex impedance curve as reference and much more in AnTune.
Here is auto port forwarding time and attenuation performed for a transmission line:

--- End quote ---

The R&S ZNL already has this feature (for $100k, you'd hope so). I was asking if there was a way to:

1) Estimate the equivalent lumped model.
AND/OR
2) What the best way to calibrate/deembed a test fixture out of a 1 port measurement, where I can only leave the other end open.

It seems like your software is doing what I described in my first post, simply taking a "reference" measurement and performing a delta measurement.

E Kafeman:

--- Quote from: Nitrousoxide on May 06, 2021, 03:05:44 am ---and it certainly has multiple parallel branches

--- End quote ---
That can be a bit hard to handle by any software and makes need for precision harder to achieve.


--- Quote from: Nitrousoxide on May 06, 2021, 03:05:44 am ---The R&S ZNL already has this feature (for $100k, you'd hope so).

--- End quote ---
To a certain degree yes but AnTune is more of an specialist tool mainly intended to handle complex wideband impedance matching, using complex source and complex load impedance and automatic matching with lossy real S-parameter components and if preferred automatic selected topology. Such things do not exist in ZNL. As best can you manually add virtual fixed ideal component values in ZNL.


--- Quote from: Nitrousoxide on May 06, 2021, 03:05:44 am --- I was asking if there was a way to:
1) Estimate the equivalent lumped model.

--- End quote ---
Yes it is possible in AnTune. Not a single button solution but four or five buttons. Up to four element can be calculated with or without user set preferred topology.


--- Quote from: Nitrousoxide on May 06, 2021, 03:05:44 am ---2) What the best way to calibrate/deembed a test fixture out of a 1 port measurement, where I can only leave the other end open.

--- End quote ---

As the unknown transmission line have several branches will all branches be included in eventual reflection.
A possibility to identify correct electric length is if short can be found by manually short-cutting with a blank PCB placed in the socket.
You can estimate short to have about same reflection attenuation and similar phase delay as for open but if high amount of precision is needed must also short parameters be measured.

coppercone2:
rebuild it with a bunch of RF relays and digital control?

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