Electronics > RF, Microwave, Ham Radio

VNA One Port Fixture De-Embedding Experiment

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Nitrousoxide:

--- Quote from: gf on May 16, 2021, 09:59:54 am ---I do not think so. I downloaded the ZNL handbook and the text reads

--- End quote ---

Whoops. I meant to say the "auto length/loss" method. That's what I get for posting while sleep-deprived.

I've built a new PCB with a significantly simplified RF path, so no worries there. It's unfortunate indeed that time gating isn't directly supported, the best I could do is emulate the first app note you linked.


--- Quote from: rf-messkopf on May 16, 2021, 10:53:05 pm ---I haven't used that function in a while, but from what I remember the "Direct Compensation" function simply works by measuring S_nn at the calibration plane (i.e., the fixture connector) at each port, with the fixture contacts open or shorted (this can be selected). Assuming reciprocity (i.e., the path from the connector to the fixture contact is the same in both directions), the square root of the measured S_nn describes the influence of one pass trough the path from connector to fixture contact. This quantity can then be applied at each port to correct for phase and loss.

This correction is applied at each frequency individually. There is no transmission line model involved, in contrast to Auto Length and Loss.

For a single port, Direct Compensation is equivalent to simple trace normalization. I.e., measure S_11^ref at the cal plane with the fixture shorted. Then the corrected measurement, up to an overall phase, is given by S_11^meas/S_11^ref.

You can do Auto Length and Loss as well as Direct Compensation with open, short, and both open and short. Due to the fringing capacity of an open a short is probably closer to ideal.

--- End quote ---

Yep. It completely zeros out the fixture, almost like I'm taking a "delta" measurement. That's basically what I figured I'd do in my original post. Also, what do you mean by "up to an overall phase"?


--- Quote from: Nitrousoxide on May 05, 2021, 09:57:33 am ---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))*.

--- End quote ---


*and also phase(S11_/S_11,ref).

So I guess really not so silly after all... I did however make a few modifications to my procedure: Ive machined a copper square to fit in the socket so I can do both open and short.


--- Quote from: Joel_Dunsmore on May 17, 2021, 01:44:33 am ---This has been done commercially for a few years using Automatic Fixture Removal (AFR for short, paid for app).  It does just what you want to do. The fixture can be characterized with just an open, or just a short, or open/short combo; or just a thru. Used all the time by RF chip makers to remove their fixture effects. 

The basic principle is discussed in chapter 11 of my book (2nd edition  www.tinyurl.com/joelsmicrowavebook ).  Basically uses some sophisticated time domain gating techniques and little special sauce. But that chapter also details Automatic Port Extension which might do the trick for you. Its a free feature on Keysight VNAs.

But the chapter also tells you how to make your own calibration kits and suss out  kit coefficients and use them for in-fixture cal.

--- End quote ---

R&S ZNL has auto port extension (estimate the phase delay and loss) built-in. It also has the provisions of defining a custom cal kit at no added cost required, something I know that you have to pay for with Keysight VNA's (I know because the mmWave guys next door keep complaining about it).

The elastomer socket is incredibly picky with the amount of force applied. If any SMD passives are required then that could potentially cause issues when I torque the socket. That and I don't have the confidence (or the tools) to measure it accurately.


--- Quote from: Joel_Dunsmore on May 17, 2021, 01:44:33 am ---Or, just for fun, I can do can AFR for you; calibrate carefully at the end of a cable using your best SMA cal kit, post an S1P file of your fixture with the port open, and I'll send you back the S2P S-parameter file of the fixture, but the measured open-data needs to go to 20 GHz, 10 MHz steps (2000 points).

--- End quote ---

I can get some data tomorrow. That would be cool to see if you could work your magic.

Wow, there are so many ways to tackle this, and as per usual, no single one is "better" than the other... The pains of engineering...

EDIT:
We also own a small copper mountain VNA, so this may be of help:
https://coppermountaintech.com/automatic-fixture-removal-plug-in/

gf:

--- Quote from: Nitrousoxide on May 17, 2021, 10:22:34 am ---*and also phase(S11_/S_11,ref).
So I guess really not so silly after all...

--- End quote ---

In the general case, you would need to measure the fixture terminated with three known impedances in order to obtain an unambiguous solution.
But if you have some a priori knowlege about the fixture, then this knownledge may help to constrain the set of possible solutions, so that fewer than three measurements may suffice.

A simple S11 response normalization resolves the ambiguity by making the a priori assumption that the fixture can be modeled by an equivalent 2-port network whose S11=S22=0. Only if this assumption happens to apply to your fixture, then the result is correct for any DUT connected to the fixture, otherwise it is not.

Nitrousoxide:

--- Quote from: gf on May 17, 2021, 01:38:06 pm ---
--- Quote from: Nitrousoxide on May 17, 2021, 10:22:34 am ---*and also phase(S11_/S_11,ref).
So I guess really not so silly after all...

--- End quote ---

In the general case, you would need to measure the fixture terminated with three known impedances in order to obtain an unambiguous solution.
But if you have some a priori knowlege about the fixture, then this knownledge may help to constrain the set of possible solutions, so that fewer than three measurements may suffice.

A simple S11 response normalization resolves the ambiguity by making the a priori assumption that the fixture can be modeled by an equivalent 2-port network whose S11=S22=0. Only if this assumption happens to apply to your fixture, then the result is correct for any DUT connected to the fixture, otherwise it is not.


--- End quote ---

The assumption is correct. I'm "looking" into the gate of a 45nm (very large) mosfet and observing how the small-signal impedance changes versus bias voltage.

Honestly, this may be easier to just perform C-V characterization. It's a shame I don't have a tool that's easily does that (considering we're a microelectroics lab, that seems a bit odd, but hey, price is a thing).

coppercone2:
my e5100 has a 'thru' calibration option. It also has an option for all 3. They recommend the thru for fast measurements, but the openshortclosed for higher accuracy. I assume they use one of these methods. Anyway, it cuts down on the work alot.

The accuracy might be in the manual.

Nitrousoxide:

--- Quote from: coppercone2 on May 18, 2021, 07:26:35 am ---my e5100 has a 'thru' calibration option.

--- End quote ---

Wouldn't 'thru' calibration in this context be irrelevant, since it's purely a 1 port device?

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