Fixed Probes

Fixed Probe Options

All probe types have a large number of options allowing you to customise how you want the graph plotted. For many applications the default settings are satisfactory. In this section, the full details of available probe options are described. Select the probe and press F7 or menu Edit Part... The following dialog will be displayed for voltage, current, power, db and phase probes:

The elements of each tabbed sheet are explained below.

Probe Options Sheet

Axis Scales Sheet

Axis Labels Sheet

This sheet has four edit boxes allowing you to specify, x and y axis labels as well as their units. If any box is left blank, a default value will be used or will remain unchanged if the axis already has a defined label.

Curve History

Curves created by fixed probes store their history which can be viewed at any time. The picture below shows how the current result and one previous result are displayed.

By default, the waveform viewer will store the current result and three previous results. How many previous results are actually displayed may be changed in a number of ways:

The stored history depth (not the displayed history depth) may be changed using the DataHistoryDepth option setting. This has a default value of 4 meaning that the current result and three previous results will be stored. Note that in some circumstances the depth will be restricted to preserve memory.

Fixed Bus Probe Options

Select device and press F7 in the usual manner. A dialog box will show similar to that shown in Bus Probe Options. But you will notice an additional tabbed sheet titled Probe Options. This allows you to select an axis type and graph in a similar manner to that described above for fixed voltage and current probes.

Using Fixed Probes in Hierarchical Designs

Fixed probes may successfully be used in hierarchical designs. If placed in a child schematic, a plot will be produced for all instances of that child and the labels for each curve will be prefixed with the child reference.

Adding Fixed Probes After a Run has Started

When you add a fixed single ended voltage or current probe after a run has started, the graph of the probed point opens soon after resuming the simulation. To do this:

Per-cycle Probes

The Per-cycle Probe specification is shown in the following table:

Per-cycle Probes
Model Name:	Per Cycle Probes
Simulator:	The device is compatible with the SIMetrix and SIMPLIS simulators
SIMetrix Parts Selector Location:	Probes > Per Cycle Voltage Probes Probes > Per Cycle Diff Voltage Probes Probes > Per Cycle Current Probes Probes > Per Cycle Diff Current Probes
SIMPLIS Parts Selector Location:	Commonly Used Parts > Per Cycle Voltage Probes Commonly Used Parts > Per Cycle Diff Voltage Probes Commonly Used Parts > Per Cycle Current Probes Commonly Used Parts > Per Cycle Diff Current Probes
Symbol Library:	None the symbol is automatically generated when placed
Symbols:
Multiple Selections:	Multiple probes can be selected and edited simultaneously.

Per-Cycle Timing Measurements

These advanced probes allow you to visualize subtle behaviour which is not otherwise easy to see. One example is to visualize how the switching frequency varies over the simulation time window. In this case, the probe first finds all edges which match the trigger conditions and then calculates a new curve with the frequency measurements on the vertical axis and the input time vector on the horizontal axis. An example waveform showing a pulse train and the per cycle frequency of the curve is shown below: The following timing measurements are supported on a per-cycle basis:

• Frequency
• Period
• Duty Cycle
• On Time
• Off Time

Per-Cycle Amplitude Measurements

The per-cycle measurement can also be applied to amplitude of an input voltage or current. For example, you can plot the mean value of a switching power converter output with the mean value being calculated on a per-cycle basis. This example is show below - the mean value of a converter output voltage is plotted in a per-cycle manner. The gate voltage is used to determine the timing edge information.

The following amplitude measurements are supported:

• Mean
• Maximum
• Minimum
• RMS
• Peak to Peak

Editing the Per Cycle Probe

Per Cycle Probe Options

The following tab allows you to define general probe options, which are explained in the table below.

Axis Scales

The following tab allows you to define the scale for the X-axis and for the Y-axis as explained in the table below.

Axis Labels

To specify axis labels and units, click the Axis Labels tab, and enter values as needed.

Note: If any box is left blank, a default value is used or remains unchanged if the axis already has a defined label.

XY Probes

XY Probes plot two curves against each other in an XY plot. Each input can be either differential voltage or inline current, and the curves can be output to any graph and axis. XY probes are available in both SIMetrix and SIMPLIS. The XY Probe specification is shown in the following table:

XY Probes
Model Name:	XY Probe
Simulator:	This device is compatible with the SIMetrix and SIMPLIS simulators.
Part Selector Location:	Probes > XY Probes
Symbol Library:	connection.sxslb
Symbols:
Multiple Selections:	Multiple probes can be selected and edited simultaneously.

Editing the XY Probe

XY Probe Options

The following tab allows you to define general probe options, which are explained in the table below.

Axis Scales

Lin/Log/Auto	Specify whether you want X-Axis to be linear or logarithmic. If you select Auto, the axis (X or Y) will be set to log if the x values are logarithmically spaced. For the Y-axis, it is also necessary that the curve values are positive for a log axis to be selected.
No Change Auto scale Defined	Controls how the axis limits are defined If you select No change, the axis scales remain as they are; this is relevant only if you are adding to an existing graph. If you select Auto scale, the axis scales are based on the minimum and maximum values of the curve. If you select Defined, enter values in the Min and Max fields.

Axis Labels

To specify axis labels and units, click the Axis Labels tab, and enter values as needed.

Note: If any box is left blank, a default value is used or remains unchanged if the axis already has a defined label.

Bode Plot Probe with Measurements

The Bode Plot Probe with Measurements generates plots for the gain and phase of the ratio of two voltages. The probe can be configured to plot only the gain or phase, or both gain and phase.

	Summary - Bode Plot Probe with Measurements
Model Name	Bode Plot Probe
Simulator	This device is compatible with both SIMetrix and SIMPLIS simulators
Schematic Menu	Probe AC/Noise > Bode Plot Probe - with Measurements
Parts Selector	Probes > Bode Plot Probe - with Measurements
Symbol Library	Connections
Model File	none
Symbol
Multiple Selections	If multiple Bode plot probes are selected before editing, all properties except the curve labels can be simultaneously changed for all probes. The curve label properties will remain unchanged for all selected probes
Usage	This schematic probe symbol plots the magnitude and phase of the ratio of two complex voltages, OUT/IN. The magnitude can be plotted in db or volts/volts; the horizontal axis scale is determined by the simulation sweep type. If the simulation is swept in log frequency steps, the horizontal axis will automatically be log scaled

Editing the Bode Plot Probe with Measurements

Bode Plot Probe - Basic Version

Connects to the input and output of a circuit to plot its gain and phase. A more sophisticated bode plot probe which provides a wide range of options along with the display of useful measurements is also available and is recommended for most applications. See Bode Plot Probe with Measurements

This basic version is useful for quick checks and for compatibility with versions 7.0 or earlier.

	Summary - Bode Plot Probe - Basic
Model Name	Bode Plot Probe - Basic
Simulator	This device is compatible with both SIMetrix and SIMPLIS simulators
Schematic Menu	Probe AC/Noise > Bode Plot Probe - Basic
Parts Selector	Probes > Bode Plot - Gain/Phase
Symbol Library	Connections
Model File	none
Symbol
Multiple Selections	If multiple Bode plot probes are selected before editing, all properties except the curve labels can be simultaneously changed for all probes. The curve label properties will remain unchanged for all selected probes
Usage	This schematic probe symbol plots the magnitude and phase of the ratio of two complex voltages, OUT/IN. The magnitude can be plotted in db or volts/volts; the horizontal axis scale is determined by the simulation sweep type. If the simulation is swept in log frequency steps, the horizontal axis will automatically be log scaled

Editing Bode Plot Probe

Fourier Analysis

A fixed Fourier probe is available which will perform a spectral analysis on a node voltage. To place this probe, select menu Probes > Place Fourier Voltage Probe.

Double click the probe to edit it. You will see this dialog box:

The settings are similar to that for random Fourier probe plotting as described here: Fourier Analysis. The documentation is repeated here for convenience.

Method

SIMetrix offers two alternative methods to calculate the Fourier spectrum: FFT and Continuous Fourier.

The simple rule is: use FFT unless the signal being examined has very large high frequency components as would be the case for narrow sharp pulses.

A description of the two techniques and their pros and cons follows.

Plot (Phase or Magnitude)

The default is to plot the magnitude of the Fourier spectrum. Select Phase if you require a plot of phase or dB if you need the magnitude in dBs.

Frequency Display

Signal Info

If the signal being analysed is repetitive and the frequency of that signal is known exactly then a much better result can be obtained if it is specified here. Check the Know fundamental frequency box then enter the frequency. The Fourier spectrum will be calculated using an integral number of complete cycles of the fundamental frequency. This substantially reduces spectral leakage. Spectral leakage occurs because both the Fourier algorithms work on an assumption that the signal being analysed is a repetition of the analysed time interval from ???MATH???t=-\infty???MATH??? to ???MATH???t=+\infty???MATH???. If the analysed time interval does not contain a whole number of cycles of the fundamental frequency this will be a poor approximation and the spectrum will be in error. In practice this problem is minimised by using a window function applied to the signal prior to the Fourier calculation, but using a whole number of cycles reduces the problem further.

Note that the fundamental frequency is not necessarily the lowest frequency in the circuit but the largest frequency for which all frequencies in the circuit are integral harmonics. For example if you had two sine wave generators of 1kHz and 1.1KHz, the fundamental is 100Hz, not 1kHz; 1kHz is the tenth harmonic, 1.1KHz is the eleventh.

You should not specify a fundamental frequency for circuits that have self-oscillating elements.

FFT Interpolation

As explained above, the FFT method must interpolate the signal prior to the FFT computation. Specify here the number of points and the order. The number of points entry may be forced to a minimum if a high stop frequency is specified in the Frequency Display section.

The number of interpolation points required depends on the highest significant frequency component in the signal being analysed. If you have an idea what this is, a useful trick to set the number of points to a suitable value, is to increase the stop frequency value in the Frequency Display section up to that frequency. This will automatically set the number of interpolation points to the required value to handle that frequency. If you don't actually want to display frequencies up to that level, you can bring the stop frequency back down again. The number of interpolation points will stay at the value reached.

If in doubt, plot the FFT twice using a different number of points. If the two results are significantly different in the frequency band of interest, then you should increase the number of points further.

Usually an interpolation order of 2 is a suitable value but you should reduce this to 1 if analysing signals with abrupt edges. If analysing a smooth signal such as a sinusoid, useful improvements can be gained by increasing the order to 3.

Data Span

Usually the entire simulated time span is used for the Fourier analysis. To specify a smaller time interval click Specify and enter the start and end times.

Note that if you specify a fundamental frequency, the time may be modified so that a whole number of cycles is used. This will occur whether or not you explicitly specify an interval.

Window

A window function is applied to the time domain signal to minimise spectral leakage (See above).

The choice of window is a compromise. The trade off is between the bandwidth of the main spectral component or lobe and the amplitude of the side-lobes. The rectangular window - which is in effect no window - has the narrowest main lobe but substantial side-lobes. The Blackman window has the widest main lobe and the smallest side lobes. Hanning and Hamming are something in between and have similar main lobe widths but the side lobes differ in the way they fall away further from the main lobe. Hamming starts smaller but doesn't decay whereas Hanning while starting off larger than Hamming, decays as the frequency moves away from the central lobe.

Despite the great deal of research that has been completed on window functions, for many applications the difference between Hanning, Hamming and Blackman is not important and usually Hanning is a good compromise.

There are situations where a rectangular window can give significantly superior results. This requires that the fundamental frequency is specified and also that the simulated signal is consistent over a large number of cycles. The rectangular window, however, usually gives considerably poorer results and must be used with caution.

Run time calculation time limit

If using the Continuous Fourier method, you can enter a maximum time limit to calculate the Fourier spectrum. If the limit is exceeded, the calculation will abort and no display will be made. The Continuous Fourier calculation time can be excessive but cannot be predicted before the simulation is started as it is necessary to know the number of simulation time points to be processed.

For the FFT method, the simulation time can be estimated to a reasonable accuracy before the simulation and this estimate will be displayed labelled Estimated calculation time:.

Arbitrary Fixed Probe

You can create an arbitrary fixed probe to plot an expression of any combination of node voltages and device currents. To use this feature, select menu Probe > Create and Place Arbitary Probe. You will see this dialog box:

Probe expression

Enter an expression to define what you wish to be plotted. Use V(nnn) to access a voltage and I(sss) to access a current. nnn and sss may be any arbitrary string starting with a letter. When you close the dialog box a symbol will be created which will reflect what you enter here. It will have single inputs named according to occurrences of V(nnn) and pairs of inputs named after occurrences of I(sss). For example if you enter the expression:

V(vin)*I(iin)

The symbol created will look like this:

The result plotted will be the product of the voltage on vin and the current in iin.

The same behaviour could be achieved using a Non-linear transfer function device (see Non-linear Transfer Function ) and a simple single ended probe. This arbitrary probe has some important advantages over that approach:

Multi-step mode

The options in the Multi-step mode group control the behaviour of the probe with multi-step analyses.

For more information about performance analysis and histograms, see PerformanceAnalysis and Histograms

Histogram options

Options for creating histograms. See above. Number of bins may be used to force a particular number of bins of the histogram. The default depends on the size of the sample and is typically half its square-root. A display of statistical calculations is displayed alongside the histogram including mean and standard deviation. The Show advanced statistics enables additional statistics such as skewness and kurtosis.

Other Arbitrary Probe Options

The other options are the same as for standard fixed probes. Refer to Fixed Probe Options

Changing Update Period and Start Delay

The update period of all fixed probes can be changed from the Options dialog box. Select menu File > Options > General... and click on the Graph/Probe/Data analysis tab. In the Probe update times/seconds box there are two values that can be edited. Period is the update period and Start is the delay after the simulation begins before the curves are first created.

Auto Probe Colouring

Fixed probes on the schematic change colour to reflect the colour of the graph curves they create. So if a red curve is created for a particular probe, the text of that probe on the schematic will change to red. This eases associating probes with plotted curves.

This colouring feature is only active for curves that are actually visible. So any probes that generate curves that are not visible or which have been deleted, will show their text in the default colour - usually black on a white background.

If the curve is highlighted in the graph window, the colour in the schematic will also reflect the highlighting.

If a probe generates more than one visible curve, the text for the probe will be shown in italics. This is the case even if all the curves are actually the same colour.