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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,

    Awesome, this is pretty incredible! Wouldn't have expected a solution for this!

    I am trying to wrap my head around the "Distortion(dB)" output for quite some time and I just do not understand it.

    Consider this simple circuit: pvccs with an IIP3 of 0dBm (in my example: c1=10 and c3=-400/3) and acmag=1mV.

    Then the Nonlinear Mag at 1st frequency is given as: a3*3/4*V^3 = (400/3) * 3/4 * 1m^3 = 100nV
    This matches perfectly the simulation. Similarly for the third harmonic: a3/4*V^3 = (400/3)/4*1m^3 = 33.3333nV

    However, "Distortion(dB)" (the second column) shows 86.86udB (!). This does not make sense to me at all!

    The Total Distortion in dB would be the squared sum of the above numbers: 10*log10(100e-9^2 + 33.3333e-9^2) = -139.5424 dB

    Even if I normalize this this by the ideal output of the fundamental in some way ((a1*V)^2 = (10*1e-3)^2 = 100e-6) the result is -99.5424 dB. I do not see how this column can be "zero dB" at all.

    If I use acmag=1, then the first column is 20.83 dB. This is close to 10*log10((100^2 + 33.333^2)/10^2) = 20.4576 dB, but since the third and last column match (and it is only a single ideal pvccs) I would expect this to match precicely as well.

    I just wanted to mention that I consulted the docs as well (spectreref.pdf, spectreRF.pdf, spectreuser.pdf, a.o.) but I did not find a description how this first column is calculated (except vague statements like "the total distortion").

    Thanks!

Reply
  • itos

    Hi Andrew & Frank,

    Awesome, this is pretty incredible! Wouldn't have expected a solution for this!

    I am trying to wrap my head around the "Distortion(dB)" output for quite some time and I just do not understand it.

    Consider this simple circuit: pvccs with an IIP3 of 0dBm (in my example: c1=10 and c3=-400/3) and acmag=1mV.

    Then the Nonlinear Mag at 1st frequency is given as: a3*3/4*V^3 = (400/3) * 3/4 * 1m^3 = 100nV
    This matches perfectly the simulation. Similarly for the third harmonic: a3/4*V^3 = (400/3)/4*1m^3 = 33.3333nV

    However, "Distortion(dB)" (the second column) shows 86.86udB (!). This does not make sense to me at all!

    The Total Distortion in dB would be the squared sum of the above numbers: 10*log10(100e-9^2 + 33.3333e-9^2) = -139.5424 dB

    Even if I normalize this this by the ideal output of the fundamental in some way ((a1*V)^2 = (10*1e-3)^2 = 100e-6) the result is -99.5424 dB. I do not see how this column can be "zero dB" at all.

    If I use acmag=1, then the first column is 20.83 dB. This is close to 10*log10((100^2 + 33.333^2)/10^2) = 20.4576 dB, but since the third and last column match (and it is only a single ideal pvccs) I would expect this to match precicely as well.

    I just wanted to mention that I consulted the docs as well (spectreref.pdf, spectreRF.pdf, spectreuser.pdf, a.o.) but I did not find a description how this first column is calculated (except vague statements like "the total distortion").

    Thanks!

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