Taking Advantage of SPICE Math Functions in Multisim 10
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Overview
Due to the increasing complexity of modern designs, many engineers choose to utilize powerful tools to aid them throughout the design process. Modern software tools aim to provide advanced functionality to meet these needs.
Multisim is an intuitive schematic capture and powerful simulation tool that continues to evolve to meet the increasing demands of the design world. With Multisim 10, National Instruments has made advancements to the breadth and sophistication of mathematical analysis. With eight new mathematical functions including RMS, Group Delay and more, simulations and analysis have become even more useful.
Why?
The new functions available in Multisim open the door for more sophisticated signal analysis and processing. From RMS voltage, to averaging, group delay integration and others, Multisim 10 provides more mathematical functionality than ever before. These math functions allow engineers and designers to quickly and easily analyze and understand an even wired variety of circuits and applications. As an example, we will consider using the new envelope detection and RMS functions to analyze an arbitrary AM (amplitude modulation) signal.
Mathematical Envelope Detection using Post-Processing Functions
When designing AM (amplitude modulation) circuits, a critical requirement is that the signal being transmitted is successfully modulated to a higher frequency. Envelope detectors are circuits that reconstruct a message from a modulated AM signal. Now with Multisim 10, you can add post-processing functions to analyses to extract the envelope of any signal.
To illustrate these new capabilities, we use the new analog behavior model (ABM) source to define an AM input voltage signal in terms of a time.
The ABM voltage source in the image below has the function:
In Multisim, the expression is entered as follows:
sin(20*pi*TIME)*0.5*cos(200*pi*TIME)*sin(5000*pi*TIME)
For more information on the ABM source see The Analog Behavior Model (ABM) Source in Multisim 10.
The image below shows the maximum and minimum envelope for the AM modulated signal. Another new function available is the RMS value of any signal. This is the Root Mean Squared value. The image below also shows this value.
Note: The envelope function requires you to input the number of points (n) on either side of a peak that the function should consider when determining the envelope. In the capture below, n is 10.
List of New Functions Added
The following is a listing and description of each of the new mathematical functions that are available in Multisim 10.
Name Description
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avg(X) |
Running average of the vector X, where
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avg(X, d) |
Running average of the vector X over d, where
if xi – d ≥ x0, otherwise
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envmax(X, n) |
Upper envelope of the vector X where n is the number of points on either side of a peak that must be less than the value for a peak to be identified.
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envmin(X, n) |
Lower envelope of the vector X where n is the number of points on either side of a peak that must be less than the value for a peak to be identified
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grpdelay(X) |
Group delay of X with results in seconds, where
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rms(X) |
Running RMS average of vector X, where
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integral(X) |
Running integral of vector X, where
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sgn(X) |
The sign or signum of a real number. It is -1 for a negative number, 0 for the number zero, and 1 for a positive number.
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How to Access the New Functions
The new math functions are accessible both through simulations as they happen and by using the postprocessor.
To add expressions from within an analysis as it occurs, choose “Simulate >> Analysis” from the main menu in Multisim, and from the “Output” Tab of the analysis dialog box, choose “Add Expression”. For a detailed explanation, please see Entering Expressions in Analyses in Electronics Workbench Multisim.
To access the post-processor choose “Simulate >> Postprocessor” from the main menu in Multisim and the Postprocessor dialog box will open. For more help on using the Postprocessor, click on the Help button from the Postprocessor dialog box.
Conclusion
In order to facilitate more sophisticated designs and analysis, Multisim 10 has added several new mathematical functions. These functions provide powerful new capabilities such as RMS measurement, envelope detection, numerical integration, and averaging. By utilizing these new functions, designers can gain a better insight and understanding of circuitry, catch design errors early in the process, and optimize key circuits resulting in a better overall design.
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