Periodic Transfer Function Analysis (pxf)
Description
A conventional transfer function analysis computes the transfer function from every source in the circuit to a single output. Unlike a conventional AC analysis that computes the response from a single stimulus to every node in the circuit, the Periodic Transfer Function or PXF analysis computes the transfer functions from any source at any frequency to a single output at a single frequency. Thus, like PAC analysis, PXF analysis includes frequency conversion effects.
The PXF analysis directly computes such useful quantities as conversion efficiency (transfer function from input to output at required frequency), image and sideband rejection (input to output at undesired frequency), and LO feed-through and power supply rejection (undesired input to output at all frequencies).
As with a PAC, PSP, and PNoise analyses, a PXF analysis must follow a PSS analysis.
Unlike other analyses in Spectre, this analysis can only sweep frequency.
Syntax
Name [p] [n] ... pxf 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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Degree of annotation. Possible values are no, title, sweep, status, steps and detailed_hb. |
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Modulation conversion parameters
The variable of interest at the output can be voltage or current, and its frequency is not constrained by the period of the large periodic solution. While sweeping the selected output frequency, you can select the periodic small-signal input frequencies of interest by setting either the maxsideband or the sidebands parameter. For a given set of n integer numbers representing the sidebands K1, K2, ... Kn, the input signal frequency at each sideband is computed as f(in)= f(out) + Ki * fund(pss), where, f(out) represents the (possibly swept) output signal frequency and fund(pss) represents the fundamental frequency used in the corresponding PSS analysis. Thus, when analyzing a down-converting mixer and sweeping the IF output frequency, Ki= +1 for the RF input represents the first upper-sideband, while Ki= -1 for the RF input represents the first lower-sideband. By setting the maxsideband value to Kmax, all 2 * Kmax + 1 sidebands from -Kmax to +Kmax are selected.
The number of requested sidebands does not change substantially the simulation time. However, the maxacfreq of the corresponding PSS analysis should be set to guarantee that | max{f(in)} | is less than maxacfreq; otherwise, the computed solution might be contaminated by aliasing effects. The PXF simulation is not executed for | f(out) | greater than maxacfreq. Diagnostic messages are printed for those extreme cases, indicating how maxacfreq should be set in the PSS analysis. In majority of simulations, however, this is not an issue, because maxacfreq is never allowed to be smaller than 40x the PSS fundamental.
With PXF, the frequency of the stimulus and of the response are usually different (this is an important area in which PXF differs from XF). The freqaxis parameter is used to specify whether the results should be output versus the input frequency (in), the output frequency (out), or the absolute value of the input frequency (absin).
You can specify the output with a pair of nodes or a probe component. Any component with two or more terminals can be a voltage probe. When there are more than two terminals, they are grouped in pairs, and you use the portv parameter to select the appropriate pair of terminals. Alternatively, you can simply specify a voltage to be the output by giving a pair of nodes on the PXF analysis statement.
Any component that naturally computes current as an internal variable can be a current probe. If the probe component computes more than one current, you use the porti parameter to select the appropriate current. It is an error to specify both portv and porti. If neither is specified, the probe component provides a reasonable default.
The stimuli parameter specifies the inputs for the transfer functions. There are two choices. stimuli=sources indicates that the sources present in the circuit should be used. The xfmag parameters provided by the sources may be used to adjust the computed gain to compensate for gains or losses in a test fixture. One can limit the number of sources in hierarchical netlists by using the save and nestlvl parameters. stimuli=nodes_and_terminals indicates that all possible transfer functions should be computed.
This is useful when it is not known in advance which transfer functions are interesting. Transfer functions for nodes are computed assuming that a unit magnitude flow (current) source is connected from the node to ground. Transfer functions for terminals are computed assuming that a unit magnitude value (voltage) source is connected in series with the terminal. By default, the transfer functions from a small set of terminals are computed. If transfer functions from specific terminals are required, specify the terminals in the save statement. You must use the :probe modifier (for example, Rout:1:probe) or specify useprobes=yes on the options statement. If transfer functions from all terminals are required, specify currents=all and useprobes=yes on the options statement.
Modulated small signal measurements are possible by using the Analog Design Environment ( ADE ) environment. The modulated option for PXF and other modulated parameters are set by the Analog Design Environment (ADE). PXF analyses with this option produce results that could have limited use outside such an environment. Direct Plot is configured to analyze these results and combine several wave forms to measure AM and PM transfer function from single sideband or modulated stimuli to the specified output. For details, see the Spectre RF User Guide.
You can define sweep limits by specifying the end points or by providing 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. In addition, 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. Alternatively, you may specify the values that the sweep parameter should take by using the values parameter. If you specify both a specific set of values and a set specified using a sweep range, the two sets are merged and collated before being used. All frequencies are in Hertz.
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