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Displaying and measuring signals I

Displaying and measuring signals I

I will demonstrate how to display and measure signals using the simple circuit in figure 1:
figure 1.png

Figure 1 Simple circuit

After running the simulation an empty waveform window will appear:
figure 2.png

Figure 2 Empty waveform window

Now click in the schematic window. The mouse cursor will change to a voltage probe. Clicking on any net with the probe will display the waveform of the voltage on that net. Similarly, clicking on a component or pin, the cursor will change to a current probe and the current through the component (or into the pin) will be displayed.
You may also analyze voltage differences by first placing a reference probe (figure 3 right, black probe) on any net, then placing the measurement probe (red) on a second net. The reference probe can be placed in two ways:
  • right click on the reference net and select "Mark Reference"
  • left click on the net to be measured, then keeping the left mouse button pressed drag the probe to the reference net. As soon as the probe is over a valid net it will change to black and the status line will show a message similar to "Release left mouse button to plot V(N001, N002)" where N001 is the net to be measured (first probe, red) and N002 is the reference net (second probe, black).
figure 3.png

Figure 3 Voltage probe and current probe

figure 4.png

Figure 4 Display of voltage waveform

Right-clicking on the name of the displayed waveform opens a new window where the waveform can be manipulated:
  • change the color of the waveform
  • attach a cursor to the waveform
  • change the waveform to another waveform or to an arithmetic expression
Changing the color needs no explanation. (otherwise go back to "getting started with Windows for dummies").
You may attach one of two cursors (1st, 2nd) or both to a waveform. In figure 5 I have attached the 1st cursor to V(n001). In addition to the cursor an additional window will open displaying the values of time and voltage at the cursor's position. Sliding the cursor along the waveform allows measurement of these values at any point of the curve.
If you use both cursors, the difference in time and value between the two cursor positions will be displayed, too. It is not necessary that both cursors be attached to the same signal. On the contrary, attaching the cursors to different signals allows you to measure differences between these signals, e.g. phase shift (as long as it's meaningful. Don't compare apples and oranges, that is e.g. voltages and currents).

Shortcut for attaching a single cursor to a signal: SHIFT+LeftMouseClick on the signal name in the waveform window.

figure 5.png

Figure 5 Measuring with the aid of a cursor

Note that in figure 5 I have combined some displays (windows) that are not shown simultaneously by LTSPICE.

You may also do some math (see also „Doing math in SPICE“). One useful application is scaling a signal by a factor to make its display better comparable to another waveform, e.g. for comparing input and output of an amplifier.
Another use of expressions with waveforms is the display of computed signals (as opposed to directly simulated signals). A more or less typical example is evaluating the instantaneous power dissipated within a component (figure 6, power dissipated by R1).

Use this in combination with the ALT+LeftMouseClick below to display the RMS power dissipated by that component.

figure 6.png

Figure 6 Power dissipated by R1

Not so obvious but helpful is the following action: ALT+LeftMouseClick on the signal name in the waveform window. This will open a window displaying the average value and the RMS value of the signal within the visible interval. Change the interval by scaling the horizontal axis as required.

figure 7.png

Figure 7 Displaying average and RMS values

The result of a transient simulation is typically a plot of currents and voltages on the y-axis over time on the x-axis. You may be interested in other characteristics as well. e.g how the voltage across a component varies with current through the component (a typical data sheet graph).
In the waveform viewer simply move the mouse over the x-axis until the cursor changes to a ruler. Left klick to se the pop-up shown in figure 8:


Figure 8 Changing the axis

Not only can you change the style (linear, logarithmic), the start and stop values (left and right in figure 8) or the spacing of the ticks, even better, by changing the "Quantity plotted" you can even use another value for the x-axis, e.g. the current through a device.

Figure 9 shows such a setup where the voltage across a diode is plotted versus the current through the diode.

Figure 9 Plotting voltage versus current

In figure 9 I have used another neat trick: By adding a plot pane (right click in the waveform viewer, select "Add plot pane") plus using the x-axis for display of the current in one pane, I can have two completely different represenations of the same simulation result withn one waveform display.

SPICE help topics to look at: waveform viewer including its sub-chapters, .MEASURE

Harald Kapp, 2014-05-12, 2014-05-26
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