Prototyping and testing
complicated electronic
circuits can be time
consuming. This
versatile package lets
you throw away the
hardware and design
and test on a computer
screen.
REVIEWED BY PETER SMITH
Multisim: for
advanced circuit
design & simulation
O
PEN ALMOST any piece of
electronic equipment these
days and chances are you’ll
see just one or two ICs, often with
hundreds of pins and only a handful
of discrete components. Usually, the
components are so small it’s difficult if
not impossible to identify exactly what
they are (resistor, capacitor, inductor,
or what?).
It’s easy to imagine the control and
precision needed to assemble these
miniature PC boards. What about the
design of the ICs themselves though
– how the heck do they design, prototype and test the circuits inside a
300-pin “mega-chip”? And how do
they make sure the ICs will work in
a real circuit before committing them
to manufacture?
Computer software, of course, is
the big answer. Ingenious software
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developers have been able to create
virtual development environments
which allow the entire design and
test phase to be carried out without a
piece of hardware in sight.
Bringing the design elements together in this way has less obvious
advantages, too. For example, hardware engineers can work at a level
of abstraction above the underlying
logic elements, greatly increasing
design speed.
In this review, we look at Multisim
V6 from Electronics Workbench, a
collection of state-of-the-art circuit
design and simulation tools.
Multisim includes all the tools
necessary to take a design from inception to finished project and as such, a
detailed review would have to cover
an enormous amount of ground. We
cannot hope to do justice to all aspects
Silicon Chip’s Electronics TestBench
of the product in this short review, so
we’ve settled on describing some of
the main features instead.
Schematic capture
Designs are drawn in a familiar
Windows environment using the
Schematic Capture module.
As with all other schematic capture
programs, Multisim has a database of
the most commonly used components
(more than 16,000 in the Power-Pro
edition) that can be placed and wired
immediately. However, Multisim’s database is perhaps unique in that every
component has a simulation model
attached to it (we look at simulation
a little further on).
If a part that you want isn’t in the
database, Multisim includes a Symbol
Editor that allows you to create your
own, either from scratch or based on
an existing component (or “symbol”).
Wiring between components is a
simple matter of clicking on the start
and end points and Multisim makes
the connection automatically. Manual
control is possible too, of course. Once
wires and components are placed,
they can be moved by clicking and
dragging.
Multisim includes a multi-level
undo feature but it performs more
like an “undelete” than an “undo”.
This means, for example, that deleted
symbols and wires can be restored but
operations like wire and component
movement cannot be undone.
Each node in the circuit is automatically assigned a unique node number
during the wiring process. Using a
feature called Virtual Wiring (“virtual”
because no actual interconnections
are shown), it is possible to connect
nodes together by manually assigning
the same node numbers. Typically, the
supply rails in a circuit are connected
in this way, resulting in less clutter
and more readability.
Readability is also one of the aims
of Multisim’s subcircuit feature. A
section or entire page of an existing
circuit can be defined as a subcircuit
and then used within another circuit.
An optional add-on module expands
the functionality of subcircuits even
further, allowing them to be saved
and edited just like any other schematic file.
Completed schematics can be exported in variety of formats to suit all
major PCB layout software packages.
However, the transition to PCB layout
is much smoother when using the
Electronics Workbench product –
Ultiboard. This is because Ultiboard
recognises information from Multisim
like component footprints and minimum track widths (gleaned during
simulation) without modification.
Fig.1: schematic entry and editing is a straightforward process. Fonts, colours
and label positions can easily be changed for a more professional look.
Fig.2: if a symbol is not in the
database, it can be created from
scratch or an existing symbol can be
modified using the Symbol Editor.
Types of simulation
As we mentioned earlier, simulation
provides a means of examining circuit
behaviour without having to physically construct it. Before we look at how
a simulation is performed in Multisim,
let’s touch briefly on the technologies
involved.
Multisim supports three different
simulation technologies – SPICE,
VHDL and Verilog.
SPICE is an analog circuit simulator, the core (or kernel) of which has
become an industry standard since
Fig.3: to access simulation model
information, it’s just a matter of
right-clicking on the component and
choosing properties. Models can be
created or imported from the model
tab.
its release to the public domain in
1972. A number of companies offer
SPICE simulators that expand on the
functionality and feature set of the
original release. A notable example
is XSPICE, which provides extensions
for digital logic simulation. Multisim
includes support for all of the most
popular SPICE extensions.
SPICE, by the way, is an acronym for
Simulation Program with Integrated
Circuit Emphasis!
VHDL and Verilog are hardware
description languages (HDLs) that are
used to both document and design
electronic systems.
VHDL was born out of a US Defence
Department contract and since its release in 1985, has been standardised
by the IEEE (Institute of Electrical
and Electronics Engineers). Verilog
started life as a proprietary hardware
modelling language and in 1990, it too
was released to the public domain and
standardised by the IEEE.
VHDL and Verilog provide a means
of designing and simulating complex
digital logic, especially Complex
Programmable Logic Devices (CPLDs)
and Field Programmable Gate Arrays
(FPGAs). Devices like our imaginary
300-pin “mega-chip” are designed
using these languages.
It is important to note that VHDL
and Verilog are behavioural level languages. They describe what a circuit’s
inputs and outputs are, what functions
are performed in the middle and how
long it all takes to happen. By contrast,
Silicon Chip’s Electronics TestBench 121
Fig.4: using Model
Makers to create
a simulation
model from the
manufacturer’s data
sheets. In this
example, we have
chosen to make
a BJT (Bipolar
Junction Transistor)
model. Model
Makers supports
many other model
“classes”, including
diodes, MOSFETs,
SCRs, op amps,
strip lines,
waveguides, etc.
when talking about digital logic, SPICE
could be described as a transistor/gate
level language.
Multisim provides simulation engines for all three of these standards
and what’s more, they can work together to co-simulate an entire mixed
mode analog and digital circuit at the
board level.
This is a big advance, as separate
simulators (often from different companies) were previously needed to
simulate mixed mode circuits – and
they rarely talked to one another!
More about models
We mentioned that all components
in the database are associated with a
simulation model. Simply put, these
models “tell” the simulator how components work.
Multisim supports SPICE, VHDL,
and Verilog models. In addition, where
a ready-defined model isn’t available,
Multisim provides a feature called
Model Makers. This feature allows
you to build an accurate simulation
model (analog or digital) directly from
the manufacturer’s data sheets. And
if that’s not enough, circuits can be
modelled at behavioural level using
the C programming language – Multisim calls this Code Modelling. Whew!
So, a simulator “knows” about components in a circuit by interpreting
their respective models. But how do
we “see” what the simulator is doing?
Simulation in action
Fig.5: view from the drivers seat – the
virtual oscilloscope.
Fig.6: this spectrum analyser costs a
lot less than its real world equivalent!
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To examine the operation of a prototype circuit we have constructed, we
would apply appropriate stimulus to
the input and view the results at the
output. In a Multisim simulation, we
do exactly the same thing, except that
all our instruments are “virtual”.
Multisim includes a whole host of
virtual instruments that function just
like their real-world counterparts.
These include an oscilloscope, spectrum analyser, logic analyser, wattmeter, distortion analyser, network
analyser, Bode plotter, function generator, word generator and of course
a multimeter.
Forget hunting for those missing test
leads – simply drop your virtual instrument of choice onto the schematic
and wire it in! Double-clicking on the
Silicon Chip’s Electronics TestBench
instrument icon brings up its display
and control panel, with mouse-activated knobs and switches.
In addition to the function generator and word generator instruments,
Multisim provides other means of
applying stimulus to your circuits.
A whole class of components called
“sources” generate AC and DC currents and voltages, as well as clocks,
pulses, one-shots, etc. Specialist AM
and FM modulated sources for radio
frequency design are also included.
The parameters for each source (such
as amplitude, frequency, etc) are
individually controllable via their
property pages.
Well, this probably all sounds just
too complex if you are a beginner
to electronics. Connecting a logic
analyser to a 2-chip counter circuit
may seem like overkill but Multisim
has the bases covered here, too. A
class of components called “indicators” provides a voltmeter, ammeter,
logic probe, hex display, lamp and
bargraph, all of which operate like
their real-world cousins. For example,
the buzzer sounds the PC speaker and
the hex display segments “light up” in
line with their logic inputs.
While simulating the high-power
audio amplifier circuit published this
month, I unexpectedly discovered that
Multisim’s fuses actually go open-circuit when their rating is exceeded. As
far as I know, Multisim doesn’t include
sound effects or burning smells (I don’t
miss them)!
Virtual components
With the circuit complete and
instruments and sources connected
and configured, it’s then just a matter
Fig.7: the logic analyser is another of
Multisim’s virtual instruments. Setting
up triggers couldn’t be simpler.
the results on a chart or graph. Types
of mathematical operations include
arithmetic, trigonometric, exponential, logarithmic, complex, vector, etc.
Programmable logic design
Fig.8: in this screen shot, we have a
virtual potentiometer (VR1) in circuit.
The properties page shows that it is increased and decreased with the “a” and
“A” keys, with each keystroke varying the value by 5%.
of hitting the simulate switch to start
the simulation running.
One of the features I really like here
is the ability to change component
values in the circuit without even
having to stop the simulation. This is
achieved by temporarily substituting
any components you would like to
vary with their “virtual” equivalents.
Virtual components (resistors, capacitors and inductors) can be increased
or decreased in value in real time by
hitting certain keys on your keyboard
– you decide which. Naturally, the
property pages for virtual components allow setting things like initial
value, percentage change with each
keystroke, etc.
Circuit analysis
We’ve talked about how Multisim’s
circuit simulator can display real-time
results on virtual instruments but it is
capable of far more. Using the SPICE
simulation engine, many different
types of analyses can be performed.
These include DC operating point,
transient, AC frequency sweep, Four
ier analysis and noise and distortion,
to name a few. The results from these
analyses are automatically graphed
and can be exported to other applications such as Excel or Mathcad.
Analyses results can be handed
to the Postprocessor module, which
performs mathematical wizardry according to your requirements and plots
Fig.9: the
Postprocessor can
act on results from
an analysis using
a variety of
mathematical
operations. The
results can then
be displayed as a
graph or table, or
simply exported to
Excel or Mathcad.
As the name suggests, programmable logic devices (PLDs) are ICs containing many logic gates (or building
blocks) which are connected at programming time to perform the desired
functions. Our imaginary “mega-chip”
could be one of these.
In order to work efficiently with
devices of this complexity, designers
describe what they want in high level
programming languages like VHDL
and Verilog.
Multisim provides a complete
development environment for PLDs.
Using the inbuilt editor, the engineer
first enters a design using the VHDL
or Verilog languages. The result is
then passed to the simulator, which is
used to examine and debug the design.
Finally, an output file is generated for
programming into the target PLD.
Note that once a PLD design is complete, it can be simulated at the board
level just like any other component in
Multisim. The engineer would simply
create a symbol for the PLD and import
the VHDL/Verilog file.
Unfortunately, a detailed look at
PLD design is beyond the scope of
this article. If you would like to know
more about VHDL or Verilog, check
out the EDA industries web page at
www.eda.org
Summary
Multisim really is an outstanding
package. It excels in the simulation
department, with features that would
make it attractive to both professionals
and educators.
Multisim is available in four editions, being Power Professional,
Professional, Personal and Education
– we reviewed the Power Professional
edition. Not all features are available
in all editions, and some tools, such
as the Ultiboard PCB layout and the
Programmable Logic Synthesis module must be purchased separately.
For further information or to order,
visit the Emona Instruments website
at www.emona.com.au or phone (02)
9519 3933.
Extensive information on the Multi
sim package can also be obtained from
the Electronics Workbench website at
www.electronicsworkbench.com SC
Silicon Chip’s Electronics TestBench 123
