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[help] pll simulation in spectre

reddevil011

I am trying to run a PSS analysis of a closed-loop PLL in spectre, but encounter   converge problems.

Is it possible to run a PSS analysis of a closed-loop PLL?

thanks! 

Parents

  • One other thing to consider is that you need to simulate the PLL as a driven circuit (i.e. not an oscillator). The fundamental frequency would be your input frequency, and the oscillator output is a harmonic of the input frequency.

    If however the variation in the VCO input has a longer period than the input frequency, this may mean that there's a dead-band in the PFD (see Ken's paper that I mentioned earlier) - or it may just mean you need to give the PSS fundamental as a sub-harmonic of the input (but this will only work if the ripple is co-periodic with the input frequency).

    It can be a challenge to get a whole PLL to converge in PSS. You almost certainly need to set the tstab so that it has locked, and even then it can be challenging. I don't see many people trying because of these challenges (and often I see fractional PLLs being used nowadays, and so you're scuppered because they're not periodic).

    Regards,

    Andrew.

Reply

  • One other thing to consider is that you need to simulate the PLL as a driven circuit (i.e. not an oscillator). The fundamental frequency would be your input frequency, and the oscillator output is a harmonic of the input frequency.

    If however the variation in the VCO input has a longer period than the input frequency, this may mean that there's a dead-band in the PFD (see Ken's paper that I mentioned earlier) - or it may just mean you need to give the PSS fundamental as a sub-harmonic of the input (but this will only work if the ripple is co-periodic with the input frequency).

    It can be a challenge to get a whole PLL to converge in PSS. You almost certainly need to set the tstab so that it has locked, and even then it can be challenging. I don't see many people trying because of these challenges (and often I see fractional PLLs being used nowadays, and so you're scuppered because they're not periodic).

    Regards,

    Andrew.

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