S-Parameter Analysis (sp)
Description
The S-parameter analysis linearizes the circuit about the DC operating point and computes S-parameters of the circuit taken as an N-port. The port statements define the ports of the circuit. Each active port is turned on sequentially, and a linear small-signal analysis is performed. Spectre converts the response of the circuit at each active port into S-parameters and outputs these parameters. There must be at least one active port statement in the circuit.
Sp analysis can output an ASCII model file that can later be read-in by a nport component. The file name and the model type can be specified by file and paramtype parameters, respectively. Currently, only paramtype=s and paramtype=y are supported, for the other possible values of paramtype, the file still generates s-parameters. The text format can be either Spectre native or Touchstone.
Spectre supports sprobes in sp analysis. Sprobe is a special testbench to allow in-situ probing of bi-directional impedances. If the 'sprobes' parameter is given, the parameters S1, S2, Z1, Z2 , StabIndex of specified sprobe will be computed and outputted to rawfile. Here S1 is the scattering parameter looking left from the sprobe; S2 is the scattering parameter looking right from the sprobe; Z1 is the impedance looking left from the sprobe; Z2 is the impedance looking right from the sprobe; StabInex is the stability Index. If sprobes and ports are given, 'ports' are used to compute normal S-param, while 'sprobes' are used to compute sprobe params.
If 'sprobeports' set, a different sprobe mode is enabled. It can calculate the complete set of network parameters for the circuit looking at the left of sprobe and the circuit looking at the right of the sprobe. In this mode, sprobe is open.
Spectre can perform AC 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 sp parameter=value ...
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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List of active ports. Ports are numbered in the order given. |
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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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If the list of active ports is specified with the ports parameter, the ports are numbered sequentially from one in the order given. Otherwise, all ports present in the circuit are active, and the port numbers used are those that were assigned on the port statements. If donoise=yes is specified, the noise correlation matrix is computed. If in addition, the output is specified using oprobe, the amount that each noise source contributes to the output is computed. Finally, if an input is also specified (using iprobe), the two-port noise parameters are computed (F, Fmin, NF, NFmin, Gopt, Bopt, and Rn).
Spectre will output file in touchstone Version 2.0 format when datafmt=touchstone2. Network data for Y- and Z-parameters in touchstone Version 2.0 files is not normalized, but in spectre and touchstone version 1.0 file, it is normalized.
If the mode parameter is set to mm, differential and common-mode S-parameters (denoted as mixed-mode S-parameters) are calculated. When mode=mm, there must be 2N, with N > 1, active port statements in the circuit. The mixed-mode S-parameters are calculated referring to the pairing of the ports, with the port numbers ordered in pair as (1,2) (3,4), and so on in the ports list. With mm, Spectre calculates differential-to-differential, differential-to-common, common-to-differential, and common-to-common S-parameters. A combination of mixed-mode and standard S-parameters is calculated if the 'mode' parameter is set to, say, m12m34s5. Then, additional differential-to-standard, common-to-standard, standard-to-differential, and standard-to-common S-parameters are calculated. In the example of mode=m12m34s5, the standard single-end port is port number 5, the two mixed-mode port pairs are (1,2) and (3,4); with Spectre placing restriction of the number on active ports to 5 given in the port list.[Mixed-Mode Order] keyword is used in touchstone Version 2.0 files to describe mixed-mode network parameters order. If mode=m12m34s5, [Mixed-Mode Order] is set to D1,2 D3,4 C1,2 C3,4 S5.
sprobeports is used to specify the sprobeport instances corresponding to sprobes. sprobeport is used to set ports in the circuit looking to the left of sprobe with portL and set ports in the circuit looking to the right of sprobe with portR'. To ensure that sprobeports setting works well even when some sprobe has no sprobeport set, set sprobeport for this sprobe to a blank string. Example:
sp sp sprobes=[spb1 spb2 spb3] sprobeports=["" spb_port2 ""]
spb_port2 sprobeport portL=[...] portR=[...]
spb_port2 is for sprobe spb2. Sprobe spb1 and spb3 have no ports setting.
When portL is set for a sprobeport, S-Parameters for ports in portL and the left port of the corresponding sprobe will be calculated. And when portR is set for a sprobeport, S-Parameters for the right port of the corresponding sprobe and ports in portR will be calculated.
When donoise=yes, two-port noise parameters will be calculated. For circuit looking to the left of sprobe, the 1st port in portL is used as in-port, the left port of sprobe is used as out-port. For circuit looking to the right of sprobe, the 1st port in portR is used as out-port, the right port of sprobe is used as in-port.
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.
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