ECD302

Tests and Troubleshooting Tools

Parameter Sweep

One of the best tools to help you better understand your circuit is the “parameter sweep” analysis.With this analysis, you can choose to vary one of your component parameters (for example, the resistance of a resistor) over a certain range to find out how changing this particular resistor value would affect your circuit output.

Low-Pass Filter

Take for example, a very simple low-pass filter:

Filename:L5_cct01.msm

AC AnalysisDoing AC analysis will tell you that this particular filter cuts-off at a frequency of roughly 165 Hz:

Find the cut-off frequency at 3 dB below the acceptance gain (in this case, it is 0 dB)

The Capacitor and the Cut-off Frequency…

Now, if I wished to find out the relationship between the capacitor value and the cut-off frequency, I could either directly change the capacitor value, and then re-do the AC analysis, or, better still, I could use the parameter sweep analysis!

Invoking the Parameter Sweep analysis…

It is under the “Simulate” menu, under “Analyses”.

Choose “Device Parameter”

We want to investigate the effects of the capacitor, so, what else can you choose?

“List” means you would like to list all the values one by one…

List here all the capacitor values you want to simulate with.

Select “AC analysis”, as that is the analysis we wish to run.

Before you click “Simulate”, you should do some further settings for your AC analysis.

As usual, we should set the vertical scale of our AC analysis as “decibel”. Unless you really want the results as linear values…

After accepting your AC analysis settings, do not forget to click on the “output variable” tab to specify your output node:

Then click “Simulate” to get the following results:

Click here for the legend.

From the output, we can see that, with smaller capacitance, the cut-off frequency is higher.

So, we may conclude that there is an proportionally inverse relationship between the capacitor value and the cut-off frequency of the low-pass filter.

Monte Carlo

No, this is not a hidden gambling module in MultiSIM.The “Monte Carlo” analysis is an analysis very similar to “parameter sweep”, whereby you can see the effects of varying a certain component parameter.

Monte CarloThe difference between “Monte Carlo” and “parameter sweep” is that, in “parameter sweep”, the user can specify the actual variation in component parameters. In “Monte Carlo”, however, the variations in component parameters is determined randomly by the software. Since the variations are determined “by chance”, it is thus named “Monte Carlo”.

Why Monte Carlo?

Why do we need Monte Carlo if we already have parameter sweep?What Monte Carlo can tell us is, for a certain circuit design that we have, if we put the design through mass production, what are the chances a finished product would have an output that deviates from our ideal output.

Common-Emitter Amplifier

Let us take for example, a simple common-emitter amplifier:

Filename:L5_cct02.msm

Ideal output

Ideally, the output should be as shown below:

Black : inputRed : output

Effect of Tolerance in Transistor Current Gain

Let us say, we are interested to find out, if the transistor 2N2222A in real life has a forward current gain, f, that is subjected to a tolerance of 20%, how that would affect the output if we mass produce the circuit using a batch of 2N2222A transistors whose f might have a 20% deviation.

Invoking Monte Carlo Analysis

Look under “Simulate”, then under “Analyses”:

Click on “Add a new tolerance”…

Choose bf for “ideal forward beta”, that is, the forward current gain

The bf value is presently 220

Choose “Gaussian” to assume that the transistor tolerance has a normal distribution. There will be further discussion of this later.

We want the tolerance in percentage, and we want it set at 20%

Do the following further settings under the “Analysis Parameters” tab:

Choose “Transient analysis” for output equivalent to that of oscilloscope (that is, the output variable versus time).

Running 25 times means we would like to simulate the production of 25 sets of the common-emitter amplifier. Each would be randomly assigned a bf value according to our earlier settings.

Choose “node 8” to simulate the voltage transient at node 8.

Then press this for further settings for the transient analysis…

With our input frequency of 100 kHz, simulating for 0.1 ms should be enough to show us the output transient for at least 10 cycles.

Upon clicking “Simulate”, you need a bit of patience to wait till all the 25 runs are finished. Then you would get the output and a report as shown below:

Result Assessment

From the results of overlapping output transient, you can see that, for all the 25 sets of supposed finished product of CE amplifier, their results are very closely similar.In fact, the worst case that happened was a 3.35% deviation of bf value, and even for that run, the result looks OK.

What does this mean?

This means the design should do just fine when put through mass production, even if the transistor beta has a tolerance of 20%.

Gaussian distribution of tolerance

But wait: didn’t we set the tolerance at 20%? How come the highest deviation is only 3.35%?Well, a normal distribution of tolerance means that possibility of a 20% deviation happening is very low compared to 2%, due to the bell shape of the possibility distribution:

That is why in the 25 runs, only a couple of runs had deviation higher than 1%.

Possibility

2% 20%Deviation