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Transient noise of a vsource component (Read 3997 times)
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Transient noise of a vsource component
Jan 06th, 2016, 6:08am

I want to perform a transient noise simulation in Cadence (because I want to include non-linear effects) but I keep getting a slight underestimation of the noise.
This is my test-case:
A vsource from analogLib, connected to a 1M resistor, which generates no noise. In the vsource, I specify 1 noise/freq pair: Freq 1 = 1, Noise 1 = 1. This should give me a flat noise PSD of 1V**2/Hz.
In the tran analysis, I specify:
Stop Time = 0.1m
Transient Noise:
Noise Fmax = 1G
Noise Update = fmax
Noise contribution = on, Instance list = the vsource component
All the rest default

The simulation runs without warnings or errors.

As a result, I plot the transient waveform at the vsource output, and from it, I calculate the noise PSD in V/sqrt(Hz):
sqrt(((rms(tran_out)**2) / VAR("BW")))
where tran_out is the transient waveform and BW is a variable for the bandwidth.

The result should be 1, as specified in the vsource. It is always smaller however. No matter which parameter I change, it is between 5-20% below the expected value.

When I increase the length of the tran simulation, I would expect that the extreme values in the transient voltage will increase, because the noise is supposed to be a gaussian process, so the probablility of larger extreme values should increase with simulation time. This does not happen however. I have a feeling that the output signal is somehow limited, which could result in a lower rms value of the noise.

The regular noise analysis does give me the correct result.

Does anyone have any idea what could cause this underestimation of the transient noise?

Thanks a lot!

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Posts: 751

Re: Transient noise of a vsource component
Reply #1 - Nov 18th, 2016, 9:54am

  The noise update method noisefmax is not recommended. What is the
value of Bandwidth? If you plot noise as a function of bandwidth what
happens? Have tried adding a shunt capacitor to define a bandwidth and
then check over the RC bandwidth. Then look at the relationship between
the RC bandwidth vs. noisefmax. It is expected that as the noisefmax
increases for a fixed RC bandwidth the noise should approach the ideal

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