R&S®Essentials | Digital oscilloscope and probe fundamentals

Understanding basic oscilloscope operation

A fundamental understanding of oscilloscopes and the basic systems is necessary for correct oscilloscope operations and valid measured data. The following will provide an overview about the settings needed to perform basic voltage versus time measurements using an oscilloscope.

What is an oscilloscope and how is it used?

The primary purpose of an oscilloscope is to measure and display voltage versus time. They are widely used for electrical / electronic designing, testing and debugging of almost anything that runs on electricity.

Oscilloscopes show voltage versus time for periodic or repeating waveforms. Modern digital storage oscilloscopes can also easily display and hold non-periodic waveforms. In addition to the basic voltage versus time display, most modern oscilloscopes often have many additional functions, e.g.:

• automatic measurement of peak-to-peak voltage or frequency
• ability to look at serial buses and mixed signal analysis
• frequency domain analysis for signals – similar to a spectrum analyzer

Basic oscilloscope operation “systems”

For measurements and to display the results, the settings of four primary “systems” are adjusted in an oscilloscope:

1) The vertical system

For measurements and to display the results, the settings of four primary “systems” are adjusted in an oscilloscope:

2) The horizontal system

For measurements and to display the results, the settings of four primary “systems” are adjusted in an oscilloscope:

3) The trigger system

For measurements and to display the results, the settings of four primary “systems” are adjusted in an oscilloscope:

4) The display system

The vertical system

The vertical axis shows voltage as a function of time. It is used to scale and position the waveform vertically. For displaying and scaling the waveforms, the volts/div control is used, which controls the amplification or attenuation of the input signal.

The most important thing to keep in mind when configuring the vertical system is to use the volts/div control to maximize the waveform on the screen. In other words, to have the positive and negative peaks as close to the top and bottom as possible without clipping the waveform.

This ensures that all bits of the scopes analog to digital converter (ADC) and the full advantages of the ADC are used. It is also easier to see small details or features in a waveform when maximizing the vertical scale.

By increasing volts/division the waveform shrinks

By decreasing volts/division the waveform grows

The position control can be used to move the waveform up or down on the screen

The horizontal system

When it comes to the horizontal system, there are two separate topics or aspects that need to be covered: waveform display and sample rate.

Waveform display

Waveform display controls in the horizontal system are related to the horizontal axis, which corresponds to time. These controls can be used to scale the waveform and/or to change its horizontal position. Similar to volts/div in the vertical system, sec/div changes the duration of each division, meaning how many cycles can be seen on the oscilloscope screen. Use the position control to move waveform right and left on the screen.

Waveform display

Sample rate

The more important aspect to the horizontal system is called sampling.
The horizontal system digitizes the input signal at a given sample rate in samples per second or at every sampling interval. These samples are stored in memory and together they make up a so-called waveform record.

The higher the sample rate:

• the greater the resolution / detail of the displayed waveform
• the greater the probability of catching infrequent events
• the greater the storage requirements (larger memory depth)

What sample rate should be chosen?
If the input signal is sampled too slowly, there is the risk of getting an alias signal that will not be an accurate representation of the sample signal.

Samples stored in memory that make up a so-called waveform record.

The Nyquist rule states that sampling should be done at twice the highest frequency to avoid aliasing. A good general recommendation is to have a sample rate that is at least 2.5 times the oscilloscope’s bandwidth.

Trigger system and trigger modes

The trigger system is extremely important, because triggering is needed for almost all oscilloscope operations. Essentially, a trigger defines the conditions that have to be met before the oscilloscope begins an acquisition or begins taking samples.
Triggering can do two different things:

First, it can stabilize a repeating or periodic signal, such as a sign wave, by causing each sweep to start at a given point on the signal

Triggers can also be used to capture not-periodic single events like a single pulse, burst etc.

It is important, to set the trigger properly. Incorrect trigger configuration is a very common problem when using oscilloscopes. There are many different types of triggers. Modern scopes can trigger on things like pulse widths, runts or glitches. The most common trigger type is the edge triggering.

In edge triggering, the trigger occurs when the threshold of the defined voltage is crossed, either on the rising edge or on the falling edge on a waveform.

Besides the different trigger types there also exist various trigger modes. The trigger mode determines the behavior of the instrument if no trigger occurs. Here we distinguish between Auto and Norm mode.

At the Auto mode the oscilloscope triggers repeatedly after a time interval if the trigger conditions are not fulfilled. If a real trigger occurs, it takes precedence. This mode helps to see the waveform even before the trigger is set. The waveform on the screen is not synchronized and successive waveforms are not triggered at the same point of the waveform.

In the Norm mode the instrument acquires a normal waveform only if a trigger occurs, that is if all trigger conditions are fulfilled. If no trigger occurs, no waveform is acquired and the last acquired waveform is displayed. If no waveform was captured before, nothing is displayed.

In edge triggering, the trigger occurs when the threshold of the defined voltage is crossed, either on the rising edge or on the falling edge on a waveform

The display system

In analog oscilloscopes the display system was little more than a cathode rate tube showing a glowing green trace. Analyzing or measuring signals often met counting divisions on the display.

Moderndigital oscilloscopeshave many display and measurement functions, such as zooming in and out of a signal and using cursors or markers to make manual measurements. There is also a large number of automated functions, like peak or peak-to-peak voltage, frequency, rise and fall times, slew rate, crest factor and pulse counts.
Many of these values can also be made on a statistical basis (statistical measurements).

Analog oscilloscope

Digital oscilloscope