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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.

Parents
  • Andrew Beckett

    Well, you might (almost) be in luck with that remaining 5%. The ac, pac and hbac analyses have some "Specialised Analyses" at the bottom of the form for performing either Compression Distortion Summary or IM2 Distortion Summary. For example:

    The idea is that for compression distortion, rather than supplying a large-signal input, it uses a perturbation method to allow a small-signal non-linear analysis with a small number of harmonics. This means that for an LNA, you can run this in AC analysis, and for a mixer you can use PAC or HBAC with just the LO applied as large signal to the mixer (so it's a single tone large-signal analysis for the PSS/HB). With the IM2 Distortion Summary you specify two tones (again can be done with AC, PAC or HBAC - so both these tones are not large signal). 

    You can specify which contributor instances you wish to compute the contribution from, and if not specified it includes all non-linear devices. The analysis is done at a single frequency, and is pretty fast (this is all part of the "Rapid IP2/IP3" type functionality - it's not dissimilar to an ac analysis with the number of points being the number of devices being analysed. 

    The analysis (for compression distortion at least, which is the one I use the most) computes the linear response from the ac (or pac) source to the output, and then computes the response with each device in turn being allowed to be non-linear (up to the number of harmonics specified; generally we suggest leaving this blank though). It then outputs the ratio of the output with the device nonlinear to the output with everything linear, and you can then (via the Results->Print menu) show the relative contribution of each device to the distortion.

    You have to specify the ac magnitude or pacmag on the source (depending on whether it's ac, pac/hbac - you will be prompted) and you should set it at a level where you are just seeing distortion.

    There isn't an IM3 Distortion Summary (if I remember rightly this was something that was technically challenging to implement, although there is an existing enhancement CCR asking for this, 1137565). However, in general you should be able to see the key distortion contributors from the normal Compression Distortion summary, it's just not shown in terms of contribution to IM3/IP3.

    More info of course in the documentation.

    Hope that helps!

    Regards,

    Andrew.

Reply
  • Andrew Beckett

    Well, you might (almost) be in luck with that remaining 5%. The ac, pac and hbac analyses have some "Specialised Analyses" at the bottom of the form for performing either Compression Distortion Summary or IM2 Distortion Summary. For example:

    The idea is that for compression distortion, rather than supplying a large-signal input, it uses a perturbation method to allow a small-signal non-linear analysis with a small number of harmonics. This means that for an LNA, you can run this in AC analysis, and for a mixer you can use PAC or HBAC with just the LO applied as large signal to the mixer (so it's a single tone large-signal analysis for the PSS/HB). With the IM2 Distortion Summary you specify two tones (again can be done with AC, PAC or HBAC - so both these tones are not large signal). 

    You can specify which contributor instances you wish to compute the contribution from, and if not specified it includes all non-linear devices. The analysis is done at a single frequency, and is pretty fast (this is all part of the "Rapid IP2/IP3" type functionality - it's not dissimilar to an ac analysis with the number of points being the number of devices being analysed. 

    The analysis (for compression distortion at least, which is the one I use the most) computes the linear response from the ac (or pac) source to the output, and then computes the response with each device in turn being allowed to be non-linear (up to the number of harmonics specified; generally we suggest leaving this blank though). It then outputs the ratio of the output with the device nonlinear to the output with everything linear, and you can then (via the Results->Print menu) show the relative contribution of each device to the distortion.

    You have to specify the ac magnitude or pacmag on the source (depending on whether it's ac, pac/hbac - you will be prompted) and you should set it at a level where you are just seeing distortion.

    There isn't an IM3 Distortion Summary (if I remember rightly this was something that was technically challenging to implement, although there is an existing enhancement CCR asking for this, 1137565). However, in general you should be able to see the key distortion contributors from the normal Compression Distortion summary, it's just not shown in terms of contribution to IM3/IP3.

    More info of course in the documentation.

    Hope that helps!

    Regards,

    Andrew.

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