eldo_user

Transient Noise Analysis Principles
Setting Up An Analysis
Transient Noise Analysis
A .NOISETRAN analysis is very similar to a normal .TRAN analysis except that the circuit
contains some additional time domain sources (current or voltage) that correspond to the
physical noise sources. Noise sources are generated as pseudo-random signals having gaussian
amplitude distribution and power spectral characteristics that correspond to the PSD model of
the noise source.
A .NOISETRAN can run an optional nominal run (normal transient run without noise sources)
and multiple runs including noise sources. The time steps and simulation tolerances are
automatically tuned to provide accurate results according to noise characteristics.
The results of the .NOISETRAN are multiple noisy runs or a RMS curve computed with
statistics from the difference between the noisy runs and the nominal run.
Applications for Transient Noise Analysis
A single noisy run may be used to compute:
• Noise PSD at the post-processing output of a circuit. This may be useful for Sigma-
Delta converters.
• A phase noise spectrum at the output of a circuit from post-processing. The targeted
circuits are free-running oscillators or PLLs. Forced circuits stimulated by periodic
signals can also be good candidates. In this particular application, multiple noisy runs
can be useful to average the phase noise results.
Multiple noisy runs are used to get the RMS value of the noisy signal function of time. This
functionality may be useful to characterize particle detectors, or CCD matrices for example.
About Simulating Multiple Noisy Runs
In some cases, you might want to run multiple noisy runs (by specifying the NBRUN parameter
of the .NOISETRAN command as > 1). Eldo can compute these noisy runs in different ways:
• Each individual run is performed sequentially, as independent transient simulations.
This mode is activated with the MRUN flag of the .NOISETRAN command. This mode
can also benefit from the distributed simulations with the .MPRUN command.
• Accelerated method (the default).
At each time step Eldo computes the nominal (noiseless) solution, and then computes
for each noisy run, a perturbation of the nominal run (in parallel). This is the default
method, and is used unless MRUN is set.
This method is much faster because it saves a lot of calculations. It considers that the
noise is a perturbation of the nominal transient simulation. This assumption is
sometimes not fully verified with some specific applications, when noise causes the
Eldo® User's Manual, 15.3 243