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problem with spectreRF

abouzied

I have the following question:

If I do iip3 simulation for an LNA and after choosing the output, the iip3 is -3dBm for example.

If I choose the net of the input as the output for iip3 calculation, it will be -3dBm(ideally it should be infinity), Why does this happen ?

I use PSS followed by PAC for simulation.

thanks in advance,

Mohamed Abouzied

Parents
  • Andrew Beckett

    Mohamed,

    The reason is that it's looking at the difference between the 1st order signal and the 3rd order product in the IP3 tree (i.e. when you have a response that looks like .II. (the "I" are the desired 1st order tones, and the "." are the IM3 products) it's half the difference between the desired and the IM3, plus the input power (which comes from the number you enter on the form, or the variable you've swept). It would only be infinite if there was no 3rd order IM3 appearing at the input port. That would only happen if you have no reflection at all and (say) an ideal voltage source. If there's an impedance there, you'll always end up with some IM3. If you plot the spectrum at the input port, you should be able to see this.

    I'm guessing a bit here, because I can't quite see what you're doing (pictures would have helped).

    Regards,

    Andrew.

Reply
  • Andrew Beckett

    Mohamed,

    The reason is that it's looking at the difference between the 1st order signal and the 3rd order product in the IP3 tree (i.e. when you have a response that looks like .II. (the "I" are the desired 1st order tones, and the "." are the IM3 products) it's half the difference between the desired and the IM3, plus the input power (which comes from the number you enter on the form, or the variable you've swept). It would only be infinite if there was no 3rd order IM3 appearing at the input port. That would only happen if you have no reflection at all and (say) an ideal voltage source. If there's an impedance there, you'll always end up with some IM3. If you plot the spectrum at the input port, you should be able to see this.

    I'm guessing a bit here, because I can't quite see what you're doing (pictures would have helped).

    Regards,

    Andrew.

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