In transient analysis there is an option for taking into account noise. Which noise components for a MOSFET are being taken into account (ie channel thermal, 1/f etc) or this is model dependent (ie BSIM4 noise parameters supplied). What exactly noise contibution is supposed to do? For example if in a resistanse i choose in generate noise->YES this gets overrided if i dont select it in noise contribution? Also in order to propagate noise over a path is it needed to select all components in path in noise contribution or not?
This is covered in the documentation, and to some extent in "spectre -h tran". If you specify noisefmax (but not noisefmin), it will compute the noise spectral density at noisefmax and then treat the noise as white at all frequencies. If you specify noisefmin, then it will include coloured noise between noisefmin and noisefmax, and the noise spectral density (at source) will be computed using the model parameters (and so will include flicker noise) - and that will control the distribution of the random noise perturbation that the noise sources in that device will produce over time.
Fundamentally the device itself needs to be generating noise - if you've turned off noise generation on the device with the "generate noise parameter on a resistor", then turning it on again in the transient noise won't help. The options on the transient noise to enable/disable particular contributors are just a way of refining the noise contributors - mainly filtering some out.
There's no need to have all devices in a path to be producing noise for the noise to propagate - you can have a noisy source and then an ideal part of the circuit and the noise will propagate still.
Does that help?
In reply to Andrew Beckett: