Signality Solutions

Modelling and simulation

Modelling and simulation can make a significant contribution to achieving your goal of right by design.

Through modelling and simulation, by the time you commit to PCB manufacture, you will already know most of how your product will behave so you spend less time debugging and face fewer - or no - PCB respins.

There are, of course, lots of reasons for not doing circuit simulation.

Here are some of them:

• I tried it once but it didn't give me a sensible answer;
• I tried it once but it didn't give me the answer I wanted;
• I tried it once but it didn't converge;
• I tried it once but it took too long;
• I tried it once but it filled up the disk and never completed;
• The simulations don't look anything like the real thing;
• The models don't work;
• I can't get the models for the devices I want to use;
• The software is too expensive;
• My circuit is too big to fit in the limits of the demo software;
• But my design is all digital and the FPGA design software does all the simulation for me;

and a particular favourite is:

• I've never had the time to learn to use it: - I'm too busy debugging my boards.

This is the classic "Catch-22":

... there's not the time to learn to use the tools effectively so they don't get used so the design is buggier than it could have been, so the time that could be spent learning the tools is used up in debugging so there's not the time to learn ...

Most of these issues stem from inexperience in modelling and simulating real circuits.

Why use simulation?

It is important to realise that simulation tools and techniques are not restricted to analogue and mixed signal circuits with transistors and opamps. Understanding the application and then applying careful modelling techniques allows the tools to be used effectively in a wide range of applications. Simulation tools can be used anywhere there are signals that can be seen as other than perfect ones and zeroes. Any signal passed from one device to another where the propagation delay along the interconnection is more than about one sixth to third of the risetime should be treated as an analogue signal. Simulation tools are vital in managing signal integrity both at the schematic and the layout stages of a design. Modelling of electro-optical devices, components and subsystems is another essential application. Simulation and switched mode power supply design go hand in hand and here the combination of high speed and high power also pushes simulation into the areas of both signal and power supply integrity.

Applied effectively, simulation is an immensely powerful tool to help reduce both the time and the risk in a design. It can be used at all stages: initial concept, design, design review, test, fault finding and failure analysis.

Clearly any method of design and checking - whether it's sums on paper, spreadsheets, Matlab scripts or simulation tools - is prone to errors. The answers are only ever as good as the data, the equations and the care put into doing and then checking the calculations. Modelling and simulation is just another way of doing these calculations. A simulator is only doing the same calculations you can do by hand or in a spreadsheet. It just takes in your data and shows you the results in a different - and in many instances a more intuitive - way.

Presenting the results in ways that are familiar and therefore more meaningful is one of the major benefits of simulation. It allows you to examine your virtual circuit using virtual test equipment and see results the same way you would with the real thing.

But this is just the start.

Modelling and simulation can take a lot of the guesswork out of circuit behaviour. You can perform tests, the results of which would be very difficult to predict without real boards being available. It also allows you to exhaustively test designs far faster and in more detail than real time breadboards and prototypes.

• The effects of component tolerances and spreads can be investigated in a matter of seconds;
• The effects of component package parasitics can be revealed that might not have been noticed until real boards were available to test;
• Signal integrity can be examined in detail;
• Power supply integrity can be interactively investigated: from the power supply design through to distribution and decoupling.
• Complex inrush current and power sequencing can be modelled.
• Custom models allow simulation of device behaviour beyond what may be available from vendor's on-line and web based tools.

All without tying up expensive and scarce test equipment resources.

Being able to test a design in this much detail means that you can start layout with much greater confidence that you will not have to make last minute schematic changes which then delay the PCB design.

This is where it starts to get really interesting.

Just because a circuit works in simulation does not mean that it will still work when it is put on a real PCB. The effects of trace impedances and coupling between them can have a significant effect on circuit behaviour. However, because modelling can include the physical interconnections between components - intentional or otherwise - it is possible to simulate many of their effects.

Since these interconnections are formed by the PCB their effects vary as the layout develops. By extending simulations to include the physical layout it is possible to check overall circuit functionality as the layout progresses. This is particularly important because you can interactively investigate the combined effects of trace and routing topologies on impedances and crosstalk to optimise the signal integrity of critical nets. In the same way, it is possible to analyse power integrity.

Using simulation in this way can further significantly improve confidence and reduce design time in the layout stage of a project.

All of which means that you can commit to PCB manufacture with a very high degree of confidence not just that the circuit will work but that it will work on that PCB.

Once prototypes arrive, not only will simulation work up to this point save time in debugging but these same simulation tools can be used alongside - and as an adjunct to - conventional test equipment.

With careful modelling to account for things like the loading and bandwidth effects of the test equipment, it is possible to directly compare simulation results with measurements made on real circuits.

It is much easier to modify a simulation than a real circuit. For example, you can look at the effect of changing the type or size of a coupling capacitor or the feedback resistor on a transimpedance amplifier in the simulation before you starting waving those nasty, hot lead-free soldering irons over your expensive PCB's.

In a simulation it is possible to check the loading effects of your test equipment by simply removing it. Not so easy to do with a real circuit and still expect to be able to see any results.

A ten times increase in your simulated oscilloscope bandwidth costs a few mouse clicks and could save the cost of hiring that 20GHz DSO just to show a glitch that your existing equipment is too slow to show you.

Furthermore, simulation can not only show you - before you build it - how your circuit behaves in normal operation. Using simulation as an integral part of Failure Modes Effects Analysis (FMEA), it can also show you how it will fail, so you can significantly improve it's safety and reliability right from the start of the design.

Whether carried out during FMEA or as a part of the normal design process, a thorough design review at both schematic and PCB layout stages are an essential part of any project.

Integrating modelling and simulation from the start of the design allows circuit operation and behaviour to be clearly demonstrated at the schematic level as part of the review process. Proposed changes can be investigated quickly and remove much of the risk often associated with alterations to designs in the later stages of development.

Modelling and simulation are not perfect.

• It is important to recognise the limitations of any tool and to use it wisely.
• There may be parts of a circuit that cannot be directly modelled. In such situations it may be possible to develop custom models or there may be ways to behaviourally model devices and functions that are perfectly valid.
• Even if you cannot model the whole of a design, the parts that you can simulate still make a big contribution in terms of time and risk reduction.  
• Simulation will not guarantee that you will get a design right first time but it will get you much closer to the ideal, much faster than if you don't use it.

With over 30 years experience in analogue and mixed signal design and over 12 years experience in circuit simulation, Signality Solutions can bring to bear a wealth of knowledge, specialist skills and techniques addressing even the most seemingly intractable problems.

If you would like to find out more about how we can help you, please contact us