Analog and digital oscilloscopes are used in a similar way. Standalone device consists of a display, a time base (with trigger settings), voltage level settings and input channels.


Image of an oscilloscope screen

Display of an oscilloscope.

On an analog oscilloscope, the display is the visible end of a cathode ray tube. The representation of the signal is done in the same way as within a tube television. A digital oscilloscope uses LCD displays and the data is provided by an arithmetic unit and shown on the display. Each display is divided into small boxes (DIVs). In X-direction the time is displayed. A box corresponds to a predetermined time, in the figure here it is 100ms. The time / Div becomes the "time base" functional unit. A change in the time base "stretches" or "compresses" the signal in width.

In the Y-direction, one box corresponds to a certain adjustable voltage level (10mV in the figure above). The level per div is displayed in the level setting. Depending on the setting the signal is "stretched" in the height.

Time Base

Sketch of the time base settings.

Time base settings.

With the time base functional unit, the measurement time is set per div. For example, if the signal frequency is 50Hz, one oscillation lasts 20ms. If you now select the time per div, e.g. 10ms, a period is shown in two divs. An important indicator is the bandwidth of the oscilloscope, i.e. up to which frequency it can still measure. The physical limit is given by the _Shannon theorem. The time bases of modern oscilloscopes range from minutes into the nanosecond range.

Use the "Trigger Level" knob to select the trigger level in terms of signal amplitude which starts one measurement. If the voltage range of the oscillation is between 0 and 1 Volt, a trigger level setting of 0V starts the waveform at 0 volts in the left part of the display. If 0.5V trigger level is selected, the waveform starts 0.5 volts, effectively shifting the signal in time. The switch "slope" determines whether the waveform is triggered with decreasing or increasing slope. Often there are even more complex options. Some devices allow a "single-shot" measurement, i.e. the first time the trigger fires, the signal is displayed only once. If you select the trigger mode "running", the signal is displayed each time the trigger level is crossed by the signal. High-quality digital oscilloscopes allow many other trigger options, such as the triggering on digital pulse sequences. Often there are further adjustment possibilities for the X-position for shifting the signal to the left or to the right.

Choosing the signal level

Voltage level settings.

Setting the oscilloscope's voltage level.

The level setting changes the voltage value per box. A signal with an amplitude of e.g. 100mV peak-to-peak (maximum to minimum) and a setting of 20mV per div results in a signal covering five divs on the display. For analog oscilloscopes there is usually one controller per channel to set the volts/div value. For digital oscilloscopes there is usually only one controller for volts/div for all channels. By pressing a button the selected channel can be set. Besides there are control knobs for the vertical displacement of the waveforms on the display, here named "Y1" and "Y2". If you have two or more channels at the same time these controls are very useful to control the waveforms, e.g. for stacking one signal on top of each other. Further options are inversion switches for mirroring the waveform in vertical direction.


Channels of an oscilloscope

Oscilloscope channels

All oscilloscopes have connections for one or more input channels. For stationary devices, these are usually located at the bottom of the front panel. BNC connectors are commonly used. SMA connectors are preferred for high-frequency oscilloscopes. You can find a variaty of adapters for switching between SMA and BNC connectors. Modern oscilloscope add further pins to the channel connectors. These pins are used by oscilloscope probes. An intelligent probe transmits the selected attenuation to the oscilloscope by these additional pins.