The noise analysis portion of WRspice performs analysis of device-generated noise for the given circuit. When provided with an input source and an output node or current, the analysis calculates the noise contributions of each device (and each noise generator within the device) to the output node voltage or current. It also calculates the level of input noise from the specified input source to generate the equivalent output noise. This is done for every frequency point in a specified range -- the calculated value of the noise corresponds to the spectral density of the circuit variable viewed as a stationary Gaussian stochastic process.
This is the classic frequency-domain SPICE noise analysis. WRspice also provides the capability of simulating thermal noise in the time domain. See the description of the tgauss ``tran'' function in 220.127.116.11 for discussion and an example.
After calculating the spectral densities, noise analysis integrates these values over the specified frequency range to arrive at the total noise voltage/current (over this frequency range). This calculated value corresponds to the variance of the circuit variable viewed as a stationary Gaussian process.
Above, out represents the output, which can be a node voltage in the standard form
v(out[,ref])or the current through a voltage source (or inductor) in one of the standard and equivalent forms
This directive initiates a noise analysis of the circuit. The parameter out is the point at which the total output noise is desired, and if this is a voltage and ref is specified, then the noise voltage v(out) - v(ref) is calculated. By default, ref is assumed to be ground. The parameter src is the name of a voltage or current source to which input noise is referred, with pts, fstart and fstop being the .ac parameters that specify the frequency range over which analysis is desired. The optional summary_pts is an integer; if specified, the noise contributions of each noise generator is produced every summary_pts frequency points.
The .noise analysis produces two plots -- one for the Noise Spectral Density curves and one for the total Integrated Noise over the specified frequency range. All noise voltages/currents are in squared units (V2 /Hz and A2 /Hz for spectral density, V2 and A2 for integrated noise).
The optional dc sweep is a dc analysis specification which will cause the noise analysis to be performed at each point of the dc sweep. The small-signal parameters are reevaluated at every sweep point, and the output vectors will be multidimensional.
In interactive mode, the noise command, which takes the same arguments as the .noise line, can be used to initiate noise analysis.
Noise analysis is not available if Josephson junctions are included in the circuit.