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About the parasitic reduction in Spectre Turbo mode

ycyang

Hi,

I am running a post-layout simulation of a frequency divider in Spectre Turbo mode. When I enable and disable the "parasitic reduction" option, the simulation results are slightly different.

For example, if the "parasitic reduction" option is enabled, the operating frequency can be as high as 3.6 GHz. While the "parasitic reduction" option is disabled, the highest operating frequency becomes 3.3 GHz.

According to the Designer's Guide Community Forum http://www.designers-guide.org/Forum/YaBB.pl?num=1291713886 the circuit behavior remains essentially unchanged by the parasitic reduction up to 30 GHz, if option "RF" is chosen.

However in my case the frequency of interest is below 5 GHz, why the simulation results differ when the "parasitic reduction" option is enabled and option "RF" is chosen?

Thanks 

 

  • Andrew Beckett

    If your clock is at (say) 4GHz (you didn't say), then you would have significant harmonics even at 30GHz. Those harmonics might be important - you didn't give any information about what you were measuring and what was different, and how different the results were. So very hard to tell.

    The idea is that the cut-off frequency specifies the frequency below which it tries to preserve poles in the transfer functions of the RC networks.

    Regards,

    Andrew.

  • ycyang

    Hi, Andrew

    My clock is at 2.8~3.6GHz and the frequency divider is a CML circuit whose modulus is from 4~7 (two cascade divide-by-2/3 cells).

    So, according to you reply, the parasitic reduction preserves poles below the cut-off frequency. Does it mean that the signals rich in high-frequency harmonic tones are not suitable to use the "parasitic reduction" option?

    Regards,

    ycyang.

  • Andrew Beckett

    Probably - although it depends on how high  you set the cut-off frequency. If you set it high enough, you'd be OK - but then of course the benefit would be limited.

    Andrew.

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