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adding white and flicker noise sources in cadence virtuoso

OneNewBoy

Hi all,

I am new to cadence virtuoso/spectre. I want to design an ADC for which I have written some components through verilog A containing their respective noise models implemented through white_noise and flicker_noise functions.

Now how to activate those sources in spectre simulation ? It's only active in small signal analysis as far as I know.
However I want noise to be present along with the main signal so that I can measure SNR etc using calculator function.

So which type of analysis to use: tran or noise ? and how (few lines of analysis statement will help)

Moreover, from cadence virtuoso manual I found that we can perform small signal analysis during transient analysis, by using below command:

  Name tran stop=stop actimes=time acnames=name

However I got a syntax error of  "ERROR (SPECTRE-16062): Output of circuit is not specified." on running below code snippet on a simple test circuit :

tran tran stop=4u write="spectre.ic" writefinal="spectre.fc" \

    annotate=status maxiters=5 actimes=[2u] acnames=[noise1]

noise1 noise start=2u stop=2.1u

finalTimeOP info what=oppoint where=rawfile

I stuffed another "info what=oppoint" statement between 2nd and 3rd line, still same error. Here my understanding is tran analysis stops at 2u and till 2.1 the simulator will run noise analysis (if wrong please correct me). I am still trying to debug this and it may be a plain syntax error. However can someone please answer above questions and guide me through the right approach.

 Thanks a lot!

Parents

  • Thanks Tawna and Andrew !

    At first I ran the transient noise. I saw the noise signature can be modulated changing noise scale factor.
    However I was not sure if my white_noise/flicker_noise sources in verilog A modules were active in this analysis, because output of a simple voltage divider circuit wasn't not much distorted (even when models of resistor thermal noise are present in verilog model) unless I gave a noise scale factor of ~ 100 say. So I got some more doubts:

     

    1. How should one decide what should be right value of noise seed and scale factors for transient noise? Or to be pessimistic should we simulate with high scale factors only ?

     

     Then I used the approach mentioned by Andrew. I deactivated transient noise part and explicitly used noise analysis after tran analysis. In the transient noise analysis I could see the spectral densities of resistor thermal noise sources:

     

    1. I think with this approach I can be sure that my thermal noise models will be used by spectre. Isn't it ?
    2. Even I am not sure if I need to use transient noise  for benchmarking ADC noise figures. Is small signal noise analysis the right approach ?
    3. Lastly, I tried to compute the SNDR at one of the nodes of voltage divider circuit, just to test simple circuit first with resistor thermal noise. I used SpectrumMeas function as mentioned by Tawna. However it gave me an error. I had selected the time domain output waveform as an input wave to this function. Any big miss with this test approach ?
    4. If yes then can someone point to ways of computing SNR etc [though I know SpectrumMeas is probably the way :)] from the output of small signal noise analysis.  Do we need to use DFT functions etc. in calculator.
    5. When we perform the small signal noise analysis, what do we have to consider before picking the time point at which to do the analysis . 

     

     Thanks a lot !!!!

     

     

Reply

  • Thanks Tawna and Andrew !

    At first I ran the transient noise. I saw the noise signature can be modulated changing noise scale factor.
    However I was not sure if my white_noise/flicker_noise sources in verilog A modules were active in this analysis, because output of a simple voltage divider circuit wasn't not much distorted (even when models of resistor thermal noise are present in verilog model) unless I gave a noise scale factor of ~ 100 say. So I got some more doubts:

     

    1. How should one decide what should be right value of noise seed and scale factors for transient noise? Or to be pessimistic should we simulate with high scale factors only ?

     

     Then I used the approach mentioned by Andrew. I deactivated transient noise part and explicitly used noise analysis after tran analysis. In the transient noise analysis I could see the spectral densities of resistor thermal noise sources:

     

    1. I think with this approach I can be sure that my thermal noise models will be used by spectre. Isn't it ?
    2. Even I am not sure if I need to use transient noise  for benchmarking ADC noise figures. Is small signal noise analysis the right approach ?
    3. Lastly, I tried to compute the SNDR at one of the nodes of voltage divider circuit, just to test simple circuit first with resistor thermal noise. I used SpectrumMeas function as mentioned by Tawna. However it gave me an error. I had selected the time domain output waveform as an input wave to this function. Any big miss with this test approach ?
    4. If yes then can someone point to ways of computing SNR etc [though I know SpectrumMeas is probably the way :)] from the output of small signal noise analysis.  Do we need to use DFT functions etc. in calculator.
    5. When we perform the small signal noise analysis, what do we have to consider before picking the time point at which to do the analysis . 

     

     Thanks a lot !!!!

     

     

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