Oscilloscope


B2.Spice's Oscilloscope allows us to examine a circuit’s function by displaying voltage over time.

Unlike a gas engine, an electronic circuit’s functioning cannot be gleaned from looking at the circuit board. Yes, the topology might be deciphered, but not the actual functioning of this particular circuit board, as a part might be blown or sitting in a cold-solder joint. With the aid of an oscilloscope, we can see a circuit’s functioning, the actual voltages and currents movements over time.

As a DSO, it has two input traces and an adjustable timebase, trigger source, and vertical signal scaling.

Each channel can be selected and edited by clicking on the Input Trace 1 or Input Trace 2 button in the Traces tab. Selecting a trace allows the signal node, the plot color and width and label to be set. (Plot widths over one pixel slow down the drawing of the waveform.) Additionally, the signal’s gain and vertical and horizontal offsets can be set. Also note that each trace can also be set as DC or AC input mode. This can play an important part in getting a signal to display correctly in the scope. Please see the note at the bottom on the difference between AC and DC mode.

The trigger source can be set to either channel 1 or 2 or another node in the circuit (Ext). The trigger mode can be set to AC or DC, and the triggering level can be adjusted in the Trigger Above and Below boxes. The Trigger Edge selects they how the signal is triggered.

The display can be frozen by pressing the “Run” or "Single", allowing complex waveforms to seen clearly and measured. The Run buttons stops the display at the point the button is pressed. The "Single" button stops the display after one complete drawing of the scope screen. Moving the vertical and horizontal sets of calipers displays the either the vertical voltage or the percentage of the oscilloscope’s left axis and the amount of time or the equivalent frequency implied along the bottom axis. The calipers can be activated in the Vertical and Timebase tabs, for vertical and horizontal calipers respectively.

*NOTE: There are two modes to the Oscilloscope - AC and DC, both for the Trace Inputs and Trigger setup. DC (Direct Coupled) shows the "real" voltage of the signal, which takes on importance if there is any DC offset. If you select the input mode as AC, a capacitor is inserted in series with the input to block out any DC signal present and pass only AC signals, which will then be centered around 0 volts This is useful if you wish to examine signals showing a small variation around one constant value. In other words, AC mode removes any offset, thus centering the signal around 0V.

This is especially important when dealing with the trigger setup. If a signal had an peak-to-peak voltage of 2V and an offset of 1V, it would be a sine wave centered around 1V. If the trigger mode was set to trigger on a DC rising edge greater than 0, nothing would show up on the scope since in DC mode, the signal's rising edge never gets to rise above 0. If you then changed the trigger mode to AC, THEN the signal would show up on the scope since the signal would then be centered around 0V and the rising edge would trigger the display.