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Noise file for Isource - Transient Simulation is NOT Noisy

PN202601231518

Hello.

Spectre (R) Circuit Simulator
Version 25.1.0.054 64bit -- 16 Jul 2025

I am running a transient simulation with an Isource and an include file of frequency/noise pairs (A^2/Hz):

1000 2.941254701486845e-10
1258.925411794167 2.696841722180617e-10
1584.893192461114 2.4836432387389e-10
1995.26231496888 2.29843362353326e-10

.........

The source will be included in a circuit of a transient noise simulation.

The simulation runs sucessfully and "sees" the noise file...but the simulation of the current has no noise; flat DC.

Please can someone help ?

***** Netlist *****

// Cell name: noise
// View name: schematic


V1v0 (v_and_i 0) vsource dc=1 type=dc


Inoise (v_and_i 0) isource dc=0 type=dc m=1  noisefile="q" isnoisy=no noisetype=noisecurrent noiseinterp=linear


simulatorOptions options psfversion="1.4.0" reltol=1e-3 vabstol=1e-6 \
    iabstol=1e-12 temp=27 tnom=27 scalem=1.0 scale=1.0 gmin=1e-12 rforce=1 \
    maxnotes=5 maxwarns=5 digits=5 cols=80 pivrel=1e-3 \
    sensfile="../psf/sens.output" checklimitdest=sqldb 


tran tran stop=10u errpreset=conservative noisefmax=200e6 noisefmin=1000 \
    noiseseed=1 write="spectre.ic" writefinal="spectre.fc" annotate=status \
    save=all maxiters=5 

Parents
  • Andrew Beckett

    Two issues:

    1. You have the current source in parallel with an ideal voltage source. The voltage at the output will always be 1V. The current could vary though
    2. You have the noise source (Inoise) with isnoisy=no - so it won't produce any noise anyway.

    If you simply change to isnoisy=yes and then plot the current through the voltage source, you'll see it's noisy.

    Or you could change the netlist to:

    rload1 (v_and_i 0) resistor r=1k isnoisy=no

    Inoise (v_and_i 0) isource dc=1m type=dc m=1 noisefile="forum202q" isnoisy=yes noisetype=noisecurrent noiseinterp=linear

    Then you'll see the voltage varying.

    Andrew

  • PN202601231518
    in reply to Andrew Beckett

    Thanks.

    I am after the noise current so of course the DC voltage is fine, I just probe noise current.

    I selected "Generate noise=yes"

    I changed my noise file to a single line:

    1e6 1e-6

    Still the same result...nothing.

    I changed the voltage source to a noiseless  resistor and I now get random voltage noise.

    So, I think, "Generate noise" for the Isource just generates random noise ??

    From the above single line noise file I would expect a single tone at 1MHz.

    Regards

    Phil

  • PN202601231518
    in reply to PN202601231518

    Correction.."generate noise=yes" always generates random noise....I can never get a single tone from my noise file.

  • Andrew Beckett
    in reply to PN202601231518

    If you looked at the current, then you would see it noisy (as I said, the voltage can't change if it's an ideal voltage source connected to the node).

    The whole point of what it has to do is to translate a frequency-domain description of wide-band noise behaviour into something that generates random noise with the same spectrum as described in the noise file (within reason). Expecting that to produce a single tone is not realistic - this is not what the noise file is for.

    Andrew

  • PN202601231518
    in reply to Andrew Beckett

    "The whole point of what it has to do is to translate a frequency-domain description of wide-band noise behaviour into something that generates random noise with the same spectrum as described in the noise file"

    That is the information I was missing; I could not find any description of this mode of operation in the documentation for the Isource.

    Thanks.

Reply
  • PN202601231518
    in reply to Andrew Beckett

    "The whole point of what it has to do is to translate a frequency-domain description of wide-band noise behaviour into something that generates random noise with the same spectrum as described in the noise file"

    That is the information I was missing; I could not find any description of this mode of operation in the documentation for the Isource.

    Thanks.

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