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Nonlinearity Separation

itos

This is with 95% a question without solution but I still try it .. maybe there is something out there.

For noise the different sources are visible in the results browser and noise summary so I can identify the devices/sources which kill noise performance.

I would be looking for something similar for linearity (e.g. IIP3, HD3, IM3, ...).

The issue: A simple differential pair (PMOS with NMOS loads or reverse).

The IIP3 versus VGS, VDS, GM/ID etc looks arbitrary and I have no control what is limiting linearity.

I can get a relatively accurate expression for IIP3 for drain current linearity but this is 30-40 dB (!!) better than what I get. So I assume I am limited by ids/vds linearity. But is it the gm transistor or the load? And what know to dial to get a handle on this?

Some sort of "nonlinearity" separation could confirm that I may neglect the common ids/vgs nonlinearity and just plot IIP3 vs. VDS of a single transistor.

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  • itos

    Hi Andrew & Frank.

    Thank you SO much for the explanation and example. I had not guessed this. It makes everything clear and I got my results matched.

    > Often I would say most people get the benefit out of the relative ranking of the contributors, rather than reading too much into the numbers themselves

    Yes, I absolutely agree on that and this is what I am interested in the end. However, before starting I want to create a baseline, understand what the numbers mean and cross-check them with other analyses before I trust them. How long did it take me to learn not to blindly trust simulator results? Specifically IIP3?

    One more question though: Is there a reason to take "(LinearOutput + Dist)/LinearOutput = 1 + Dist/LinearOutput" rather than "Dist/LinearOutput"? This results in a very unnatural feeling in me because the number is so close to one. And when else do you see udB? Something like "Dist/LinearOutput" would be way more natural and expected. And as a bonus, it would be identical to IM3 (for sufficiently simple setup).

    Not least - this is one reason why for very small noise figures one tends to use noise temperature rather than noise figure, because the number would be so close to one.

    Similarly: What is the reason to only take the the distortion at the fundamental into account and not all harmonics?

    Often I would say most people get the benefit out of the relative ranking of the contributors, rather than reading too much into the numbers themselves - See more at: community.cadence.com/.../36868
    Often I would say most people get the benefit out of the relative ranking of the contributors, rather than reading too much into the numbers themselves - See more at: community.cadence.com/.../36868
    Often I would say most people get the benefit out of the relative ranking of the contributors, rather than reading too much into the numbers themselves - See more at:
    Nonlinearity Separation
    This is with 95% a question without solution but I still try it .. maybe there is something out there. For noise the different sources are visible in the results browser and noise summary so I can identify…
    By in Cadence Technology Forums > Custom IC Design
    9 replies
    Latest by
    How long did it take me to learn not to blindly trust the IIP3 output ... ?

    Both questions are just out of curiosity - my question itself has been answered!

Reply
  • itos

    Hi Andrew & Frank.

    Thank you SO much for the explanation and example. I had not guessed this. It makes everything clear and I got my results matched.

    > Often I would say most people get the benefit out of the relative ranking of the contributors, rather than reading too much into the numbers themselves

    Yes, I absolutely agree on that and this is what I am interested in the end. However, before starting I want to create a baseline, understand what the numbers mean and cross-check them with other analyses before I trust them. How long did it take me to learn not to blindly trust simulator results? Specifically IIP3?

    One more question though: Is there a reason to take "(LinearOutput + Dist)/LinearOutput = 1 + Dist/LinearOutput" rather than "Dist/LinearOutput"? This results in a very unnatural feeling in me because the number is so close to one. And when else do you see udB? Something like "Dist/LinearOutput" would be way more natural and expected. And as a bonus, it would be identical to IM3 (for sufficiently simple setup).

    Not least - this is one reason why for very small noise figures one tends to use noise temperature rather than noise figure, because the number would be so close to one.

    Similarly: What is the reason to only take the the distortion at the fundamental into account and not all harmonics?

    Often I would say most people get the benefit out of the relative ranking of the contributors, rather than reading too much into the numbers themselves - See more at: community.cadence.com/.../36868
    Often I would say most people get the benefit out of the relative ranking of the contributors, rather than reading too much into the numbers themselves - See more at: community.cadence.com/.../36868
    Often I would say most people get the benefit out of the relative ranking of the contributors, rather than reading too much into the numbers themselves - See more at:
    Nonlinearity Separation
    This is with 95% a question without solution but I still try it .. maybe there is something out there. For noise the different sources are visible in the results browser and noise summary so I can identify…
    By in Cadence Technology Forums > Custom IC Design
    9 replies
    Latest by
    How long did it take me to learn not to blindly trust the IIP3 output ... ?

    Both questions are just out of curiosity - my question itself has been answered!

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