4.1 Instrumenting a circuit simulation in national instruments

Instrumenting a simulation in multisim

Virtual instruments are components in Multisim which model real-world bench-top instruments. Examples of virtualinstruments in Multisim include Oscilloscopes, Function Generators, Network Analyzers, and Bode Plotters.

Virtual instruments provide designers with an easy and intuitive method for interacting with their circuits asthey would in a testing or prototyping phase.

In addition to the provided virtual instruments, designers with familiar with National InstrumentsLabVIEW can create their own custom instruments from scratch. For example, one could create a custom noise generator to modelelectromagnetic interference.

Custom virtual instruments written in NI LabVIEW can also acquire real-world data and use the data to drivesimulations, and can send generated data to analog output hardware, allowing simulated data to control real-world devices. The LabVIEWdevelopment suite is required to create custom LabVIEW instruments, but not necessary to run existing LabVIEW based instruments.

To place a virtual instrument, select the desired instrument from the Instruments toolbar (Figure 1). To viewthe front panel of the instrument, double-click on the instrument icon. Make connections to the terminals of the instrument icon asyou would to any other component.

Multisim also provides designers with simulated benchtop instruments. These are instruments such as theTektronix TDS 2024 Oscilloscope. They will look and operate exactly according to the manufacturers’user manuals.

A single circuit can have multiple instruments attached to it, including (for most tiers) multiple instances ofthe same instrument. In addition, each circuit window can have its own set of instruments. Each instance of any instrument isconfigured and connected independently.

An overview of the more common instruments is included in this section. For more detailed information on how touse each instrument, consult the Multisim User Guide or helpfile.

Multimeter

Use the multimeter to measure AC or DC voltage or current, and resistance or decibel loss between two nodes in acircuit. The multimeter is auto-ranging, so a measurement range does not need to be specified. Its internal resistance and currentare preset to near-ideal values, which can be changed.

Function generator

The function generator is a voltage source that supplies sine, triangular or square waves. It provides aconvenient and realistic way to supply stimulus signals to a circuit. The waveform can be changed and its frequency, amplitude,duty cycle and DC offset can be controlled. The function generator's frequency range is great enough to produce conventionalAC as well as audio- and radio-frequency signals.

The function generator has three terminals through which waveforms can be applied to a circuit. The commoncenter terminal provides a reference level for the signal.

Oscilloscopes

Multisim includes several varieties of oscilloscopes. All of the scopes can be controlled like real-worldoscilloscopes. Horizontal timing and vertical voltages parameters can be adjusted. Triggering levels, and types are also selectable.Data from the Multisim-exclusive oscilloscopes is available in the Grapher after the interactive simulation has completed by selectingView/Grapher.

Multisim provides the following oscilloscopes:

• 2-Channel.
• 4-Channel.
• Agilent 54622D Mixed Signal Oscilloscope.
• Tektronix TDS 2024 Four Channel Digital Storage Oscilloscope.

Bode plotter

The bode plotter displays the relative phase or magnitude response of an input and output signal. This isespecially useful when characterizing the bandwidth of filter circuits.

Spectrum analyzer

The spectrum analyzer is used to measure amplitude versus frequency. This instrument is capable of measuringa signal's power and frequency components, and helps determine the existence of harmonics in the signal.

The spectrum analyzer displays its measurements in the frequency domain rather than the time domain.Usually the reference frame in signal analysis is time. In that case, an oscilloscope is used to show the instantaneous value as afunction of time. Sometimes a sine waveform is expected but thesignal, rather than being a pure sinusoidal, has a harmonic on it. As a result, it is not possible to measure the waveform's level. Ifthe same signal is displayed on a spectrum analyzer, its amplitude is displayed, along with its frequency components, that is, itsfundamental frequency and any harmonics it may contain.