De-embedding process of a test-fixture

[Henry797]

De-embedding is the act of taking data that is measured in a test fixture and removing the effects of the fixture so that the data is accurate to reference planes that are more useful, using vector measurements of known standards. For example, you can de-embed a FET measurement so that the reference planes are the gate and drain busbars; these data can be used in an amplifier design or to create an equivalant ciruit model.
I know the definition of de-embedding and kind of understand how it could be used in a transmission line to account for the losses involved in a test fixture while measuring the transmission of a DUT (might be a SMT device).
Now my question is what does de-embedding process account for? Is it only the attenuation due to the test fixture involved? Or can we extract parameters such as the dielectric or conductivity of a DUT in a transmission line using this? If so, can someone suggest me a paper or a previous work done on this? And can anyone tell me if de-embedding can be done using simulation software such as HFSS? or is it that this process can only be performed in practice (while measuring)

 

[PlanarMetamaterials]

De-embedding can definitely be done in HFSS (in post-processing, nonetheless).

In my understanding, de-embedding removes the effect of the propagation constant of your transmission-line over a certain distance. This will affect both magnitude (the decay constant alpha) and phase (the phase constant β).

Of course, this assumes that the transmission-line is uniform all the way from the port up to the specified distance, so it is your responsibility to ensure that this criteria is met.

 

[Henry797]

Thank you for your reply.
What I understood from your statement is that by referring the transmission line as uniform, are you trying to say that there will be no effect of the test fixture, or no other losses from the test fixture? And also by removing the propagation constant to a certain distance, what can be achieved from it? I may not have understood why, but could you perhaps elaborate a little bit on how, removing the attenuation and phase from the signal is helping. Is it a comparsion? if so, what is it giving me?
Many Thanks

 

[PlanarMetamaterials]

What I mean to say is that if you have a DUT connected to one or more uniform transmission lines, you typically measure at the end of these lines. The obtained results at these ports include phase shift and attenuation due to the transmission lines. By de-embedding, you get rid of these transmission-line effects and only measure the effects of the DUT.

 

[Henry797]

Hi,
Absolutely agree with you on that. Now can you also tell me the true meaning of de-embedding? If I say I want to de-embed some parameters off a DUT in a transmission line (say microstrip line) would I be using the term de-embedding correctly?
And also in the latter case, if the DUT is a block of mylar film (or glass perhaps) between the transmission line, if I want to de-embed the DUT, by doing so, I'm only getting rid of the phase or attenuation of the wave due to the right or left part of the trace on either side of the DUT. And hence with the information (S21) left after de-embedding, I can extract the physical parameters of the DUT?

 

[PlanarMetamaterials]

Hi Henry797,

Where I come from, we discuss de-embedding as a method of port objects. So I would say "I de-embedded the ports to 25mm". I would not use the term "de-embedding" as some sort of substitute for "measuring". De-embedding is a process applied after measurement.

Yes, you are correct. If I have the scattering parameters after de-embedding, they will be the parameters of the DUT only and do not include the effects of the transmission line.

 

[Henry797]

Hi PlanarMetamaterials,
Many Thanks for your reply! It was indeed very helpful.
Just one more question to ask, and also would like to request to excuse my ignorance, but if de-embedding process this efficient, in other words we're being able to extract the transmittance of the MUT( Material under test) alone, using a microstrip transmission line, does this mean we can extract physical parameters such as refractive index and hence the dielectric of the material ? Won't it account for any other losses? Just wanted to know this since you seem to know a lot about de-embedding process. But if so, why isn't this process used for measuring the material characteristics? If known any publications on this process, could you perhaps share few here.
Many Thanks
Henry797

 

[PlanarMetamaterials]

Hi Henry797,

I'm not sure how you would do material parameter extraction without the de-embedding process. As far as I'm aware, de-embedding has always been associated with retrieval of material parameters. See for example (found with a quick google search) https://web.mst.edu/~marinak/files/My_publications/Papers/IEEE_EMC_2009_Abhilash_publ.pdf - specifically section D Figure 5.

 

[Henry797]

Hi PlanarMetamaterials,
Thankyou for the paper. I wasn't aware that material extraction was possible using microstrip transmission line, as I've always done free space transmission for extracting material characterestics. Good to know a different paradigm of measurement for extracting physical parameters of materials.
Kind Regards
Henry797