Oscilloscope Maths Functions in Proteus VSM

Oscilloscope maths functions in Proteus VSM allow you to add or subtract the signals from two different channels. A typical application would be displaying the voltage waveform across a component which is not grounded at either end.

As an example, the 4-channel oscilloscope shown below measures the voltage between resistor R2 and an internal ground, but it is not so easy to measure the voltage across a component if neither end is grounded, as in R1.

In this case, even though the Channel A oscilloscope probe is attached to the upper end of R1, the voltage displayed is that across R1 and R2 combined. We actually need to subtract the Channel B signal from Channel A in order to produce the voltage across R1 alone.

A look at the oscilloscope controls shows that we can add Channels A and B (or C and D), but not subtract.

However, we can combine the Invert function on Channel B with A+B button in to find the voltage across R1, since A + (-B) = A - B.

Practical Maths

Okay, so let's see a more practical application of this feature. Consider a series R-L circuit, as shown below:

In this case, the current through the inductor should lag behind the voltage across the inductor by 90°. The voltage across the resistor will be in phase with the current through the resistor, but we also know that the same current flows through each component, since this is a series circuit.

To see the voltage across L1, simply connect Channels A and B as seen above, and then select options to subtract Channels A and B. Channel C is used to display the voltage across R1, which as discussed, has the same phase relationship as the current through both L1 and R1. Resulting waveforms are shown below.

The magenta waveform representing the current clearly lags behind the yellow waveform, which is the calculated voltage across L1. The phase relationship may be quickly estimated by counting the green squares on the oscilloscope display as Phase Difference / Periodic Time × 360°, or 3 / 12 × 360 which gives the expected 90°.

Summary

Mathematical oscilloscope functions can be really useful when you need to determine phase relationships in AC circuits, such as R-L, R-C, or even R-L-C. The 4-channel oscilloscope in Proteus VSM allows up to two calculated signals to be displayed, four standard signals, or any combination. And of course, you can use more than one oscilloscope if necessary.

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