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Transient Noise Analysis (fmin)

em1rk

Hello,

I try to set the fmin via Tran Noise Options to 1Hz. However, fmin is limited to 1/simulationTime no matter what I set. First of all, if I run AC noise analysis, I know that within the frequency band < 250Hz (which is 1/simulationTime), there is noise that adds up to the final value. I also would like to match the Tran noise result to AC noise result. For this reason I aim to simulate the noise in frequencies < 250Hz which is not possible for TRAN simulations unless I set the simulation time as 1s instead of 4ms.

I expect that if I run the simulation with multiple runs with simulation time equals 4ms, I should see the low frequency component independent of the simulation time (if fmin was indeed equal to the value I set as 1Hz). I expect that if I take the histogram of the signal (noise) at a certain time moment, the standard deviation should also correspond to the low frequencies. I believe I dont have to wait 1seconds to observe noise @ 1Hz. 

Can you please help me understand where I am making a mistake unless the tool is not suited to set fmin (even if you set) to a value < 1/simulationTime ?

Thank you in advance.

PS: I have seen this entry Transient Noise Simulation for Different input/output frequencies - Custom IC Design - Cadence Technology Forums - Cadence Community and response of Andrew below:

Andrew Beckett statement: That said, we now (in recent versions - IC617 together with SPECTRE161) set noisefmin to 1Hz by default rather than asking the user to enter it. This caused confusion and only saved a little simulation time with the improvements made in the analysis over recent years. So the noise you observe is going to be based on the duration of the simulation. 

The version I use is "@(#)$CDS: virtuoso version 6.1.8-64b 07/11/2023 19:24 (sjfhw317) $"

Best regards,

Emir

  • Emir,

    If you think about it, the effect of noise at frequencies lower than the stop time can't be (properly) observed - simply because you'd only have a fraction of the period for that noise in the simulation. That's essentially a consequence of the Nyquist–Shannon sampling theorem. With small-signal noise analysis, that's done in the frequency-domain - but for transient you have to be able to run the simulation long enough in time to be able to observe the noise...

    Andrew

  • in reply to Andrew Beckett

    Hello Andrew,

    Thanks for your message. I think I get your point. Maybe my question becomes more theoretical rather than tool related right now. I have a node where I measure the noise which is reset periodically.  After RESET is released I have an integration of current on a capacitor and I measure the standard deviation of the capacitor voltage. Can it be concluded that then the actual fmin should be set as the integration time ? The value I read is the standard deviation from a multiple run TRAN simulation at the end of integration. And the integration starts after RESET is released on that node.

  • in reply to em1rk

    I'm not entirely sure what you're asking. You shouldn't set noisefmin yourself (there's no need, unless the simulation is longer than 1s, I think). What do you mean by the "integration time" here?

    Andrew

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