Noise Analysis (noise)
Description
Noise analysis linearizes the circuit about the operating point and computes the noise spectral density at the output. If you identify an input source, the transfer function and the input-referred noise for an equivalent noise-free network are computed. If the input source is noisy, the noise figure is also computed.
The noise is computed at the output of the circuit. The output is specified with either a pair of nodes or a probe component. To specify the output of a circuit with a probe, specify it with the oprobe parameter. If the output is voltage (or potential), choose a resistor or a port as the output probe. If the output is current (or flow), choose a vsource or iprobe as the output probe.
If the input-referred noise is desired, specify the input source by using the iprobe parameter. Currently, only a vsource, an isource, or a port may be used as an input probe. If the input source is noisy, as is a port, the noise analysis computes the noise factor (F) and noise figure (NF). To match the IEEE definition of noise figure, the input probe must be a port with no excess noise and its noisetemp must be set to 16.85C (290K). In addition, the output load must be a resistor or port and must be identified as the oprobe.
If port is specified as the input probe, both input-referred noise and gain are referred back to the equivalent voltage source inside the port. S-parameter analysis calculates those values in traditional sense.
The noise analysis always computes the total noise at the output, which includes contributions from the input source and the output load. The amount of output noise that is attributable to each noise source in the circuit is also computed and output individually. If the input source is identified, the input-referred noise is computed, which includes the noise from the input source itself. Finally, if the input source is identified and is noisy, the noise factor and noise figure are computed. Therefore, if:
Ns = noise at the output due to the input probe (the source)
Nl = noise at the output due to the output probe (the load)IRN = input referred noise
When the results are output, No is named out, IRN is named in, G is named gain, F is named F, and NF is named NF.
Spectre can perform Noise analysis while sweeping a parameter. The parameter can be frequency, temperature, component instance parameter, component model parameter, or netlist parameter. If changing a parameter affects the DC operating point, the operating point is recomputed at each step. You can sweep the circuit temperature by giving the parameter name as temp, without a dev or mod parameter. In addition, you can sweep a netlist parameter by giving the parameter name without a dev or mod parameter. After the analysis is complete, the modified parameter returns to its original value.
Syntax
Name [p] [n] noise parameter=value ...
The optional terminals (p and n) specify the output of the circuit. If you do not specify the terminals, you must specify the output with a probe component.
Parameters
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Use the operating point computed in the previous analysis. Possible values are no and yes. |
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Frequency when a parameter other than frequency is being swept. |
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Compute total noise at the output defined by this component. |
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File that contains an estimate of the DC solution (nodeset). |
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DC operating point output file at the first step of the sweep. |
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DC operating point output file at the last step of the sweep. |
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Restart the DC solution from scratch if any condition has changed. If not, use the previous solution as an initial guess. Possible values are no and yes. |
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Degree of annotation. Possible values are no, title, sweep, status and steps. |
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You can define sweep limits by specifying the end points or the center value and span of the sweep. Steps can be linear or logarithmic, and you can specify the number of steps or the size of each step. You can specify a step size parameter (step, lin, log or dec) to determine whether the sweep is linear or logarithmic. If you do not specify a step size parameter, the sweep is linear when the ratio of stop to start values is less than 10 and logarithmic when this ratio is 10 or greater. All frequencies are in Hertz.
The small-signal analysis begins by linearizing the circuit about an operating point. By default, this analysis computes the operating point, if it is not known, or recomputes it if any significant component or circuit parameter has changed. However, if an operating point was computed during a previous analysis, you can set prevoppoint=yes to avoid recomputing it. For example, if prevoppoint=yes and the previous analysis was a transient analysis, the operating point is the state of the circuit at the final time point.
Nodesets help find the DC or the initial transient solution. You can specify nodesets in the circuit description file with nodeset statements or in a separate file by using the readns parameter. When nodesets are specified, Spectre computes an initial guess of the solution by performing DC analysis, while forcing the specified values on to nodes by using a voltage source in series with a resistor whose resistance is rforce. Spectre then removes these voltage sources and resistors and computes the required solution from this initial guess.
Nodesets have two important uses. First, if a circuit has two or more solutions, nodesets can bias the simulator towards computing the required solution. Second, this is a convergence aid. By estimating the solution of the largest possible number of nodes, you might be able to eliminate a convergence problem or significantly speed up convergence.
When you simulate the same circuit multiple times, it is recommended that you use both write and readns parameters and assign the same file name to both parameters. DC analysis then converges quickly even if the circuit has changed since the last simulation, and the nodeset file is automatically updated.
During the initial operating point DC analysis, you may force certain circuit variables to use the values given in the ic file, ic statements, or ic parameter on the capacitors and inductors. The ic parameter controls the interaction of various methods of setting the force values. The effects of individual settings are as follows:
force=none:All initial conditions are ignored.
force=node:The ic statements are used, and the ic parameters on the capacitors and inductors are ignored.
force=dev:The ic parameters on the capacitors and inductors are used, and the ic statements are ignored.
force=all:Both ic statements and ic parameters are used, with the ic parameters overriding the ic statements.
If you specify an ic file with the readforce parameter, force values from the file are used and any ic statements are ignored.
After you specify the initial conditions, Spectre computes the DC operating point with the specified nodes forced to the given value by using a voltage source in series with a resistor whose resistance is rforce (see options).
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