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PNoise Infinite flicker noise message

Kenny Lu

Dear all,

     I am simulating a chopper opamp circuit using the pss/pnoise analyses. I would like to see the effect of flicker noise being up-modulated to the chopping frequency. My chopping frequency is set at 1 MHz, which is set as the pss fundamental frequency. Please see below for my simulation setup:

Spectre Version: 23.1.1.362.isr5

After I ran the pnoise simulation, this was the output spectrum I acquired. As shown below, the output spectrum remains flat at 1MHz instead of showing a flicker noise peak there.  When I checked the simulator output log, there were messages saying infinite flicker noise ignored, as shown in the second figure below.

Is there something wrong with my settings? Is it normal to see this message in the output log?

 

Thank you very much

Kenny

  • If you want to see the flicker noise around the fundamental frequency, you should run the pnoise analysis with "Sweeptype: relative" and "Relative Harmonic: 1". With your current setup, the frequency points around 1 MHz are not close enough and the infinite noise at 1 MHz (just like at 0 Hz) is ignored as reported by Spectre.

  • in reply to Frank Wiedmann

    Hi Frank,

         Thank you very much for your reply. I have just tried SweepType=relative, relharmnum=1 settings as shown below:

         

         However the same message still appeared in the simulator output log and the waveform was still flat around the fundamental frequency.

    Thank you

  • in reply to Kenny Lu

    What does the output log say about the simulated frequencies? Can you show the waveform plot?

  • in reply to Frank Wiedmann

    Hi Frank,

         Please refer to the figures below for the simulator output log and the simulated PNoise waveform.

    Simulation Setup:

    Simulator Output Log:

    Simulation Waveform:

    Not sure if 0 Hz at relative frequency scale = 1 MHz??

    Thank you very much

    Kenny

  • in reply to Kenny Lu
    Kenny Lu said:
    Not sure if 0 Hz at relative frequency scale = 1 MHz??

    That should indeed be the case. Please take a look at the header of the pnoise analysis in the output log (which you unfortunately didn't show in your screenshot). What does it say after "freq = "? When you compare the results for relharmnum=1 with your earlier results (where you did an absolute frequency sweep), you should also see that they are different.

  • in reply to Frank Wiedmann

    Hi Frank,

         Thank you very much for your help. I appreciate it. Please see below for what I noticed in the output log:

    Since I don't know why I cannot post screen shots, I will simply type the message here.

    PNoise with relharmnum=1:

    **********************************************************************************

    Periodic Noise Analysis 'pnoise': freq = 1 MHz + (10 mHz -> 100 MHz)

    ***********************************************************************************

    Therefore, the frequency at 0 = 1 MHz. The peaking is displayed at 0 Hz, showing that flicker noise has been modulated to here.

    Is my understanding correct?

    Thank you

    Kenny

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