What every electronics engineer should know about oscilloscopes

Of all the test equipment that electronics engineers and technicians have at their disposal, the most useful is undoubtedly the oscilloscope. Oscilloscopes are powerful tools that allow electronics engineers and technicians to quickly and accurately capture time-varying measurements of voltage (or other parameters such as current) that cannot be easily accomplished with any other device in the lab.

Oscilloscopes are widely used in manufacturing, circuit design and other industries as a basic and essential tool for troubleshooting, signal integrity and simply understanding how electronic circuits work.

Although the buttons, knobs, probes, and related probe accessories and color display screens of modern oscilloscopes seem very complicated, which makes people who want to learn feel discouraged. But in fact, don't be scared by the complex appearance of the oscilloscope. As long as you master some basic skills, it is a very simple and easy-to-use device. Moreover, the current advanced oscilloscopes also have full touch functions, which means that all the original complex button knob operations can be replaced by touch screen operations. If you have experienced the early button phones and the current smart phones, you will quickly understand the advantages of touch screens.

The key to becoming an oscilloscope expert is to first understand the basics of oscilloscopes, and then use this basic knowledge to expand your learning. The following brief article will introduce some key points and common problems that some new users encounter in the basic use of oscilloscopes. This will help us quickly understand oscilloscopes and find the right direction for learning. As the learning time lengthens, after using the oscilloscope for a certain period of time, we can almost perform any measurement, and eventually become more skilled and professional in the field of testing.

For the sake of simplicity, this article only takes the Microsignal STO1104C smart oscilloscope with full touch and Android system as an example. As for the old traditional analog oscilloscope, it has been obviously eliminated with the progress of the times, so it will not be repeated.

Grounding and safety

Before understanding the basics of oscilloscopes, we first need to understand proper grounding and safety of oscilloscopes to avoid personal injury and damage to the oscilloscope or any accessories connected to the oscilloscope. Improper connection of the probe will form a current path, thus damaging the probe. To avoid electric shock, the oscilloscope must be connected to the earth through a grounding wire. In short, the metal part of the probe connected to the oscilloscope is directly connected to the safety ground through the power cord of the oscilloscope.

You can try changing the connection yourself with an ohmmeter. This is a low impedance connection, and when the circuit being measured is also connected to earth, a loop is formed, and the very low impedance causes excessive current to flow through the circuit. The current carrying capacity of the probe's ground lead quickly exceeds its rating, the lead suddenly breaks, and you may hear a loud pop!

The best way to fix this is to break the ground loop by isolating the circuit under test or isolating the oscilloscope ground. If the oscilloscope's safety ground fails, your best option is to make sure the circuit under test is not connected to the earth safety ground. Use an isolated oscilloscope or differential probe, or choose to power the test circuit with an isolated power supply or battery. Be careful when using something like a USB connector to power the circuit under test, as this type of equipment is not usually isolated from ground and will still experience problems with ground loops.

What is an oscilloscope?

An oscilloscope can measure the voltage waveform of the measured signal through a voltage sensor (i.e. the most common oscilloscope voltage probe) or some other sensors (such as pressure sensors, current probes, noise meters, etc.). The curve graph produced by the oscilloscope measures the voltage on the vertical axis and represents the signal time on the horizontal axis. From the captured waveform, we can obtain data such as the signal's frequency, amplitude, period, phase, distortion, noise, DC, AC, duty cycle, rise/fall time, etc.

Base

In addition to the display, there are three other important features that make up an oscilloscope. These features are the oscilloscope's trigger, the volts per vertical division, and the time per horizontal division.

trigger

The trigger function is used to synchronize the horizontal scan of the signal, which is essential for us to observe the signal conveniently. The trigger makes the repeated waveform look still on the display by repeatedly displaying the triggered part of the input signal. The most basic and common trigger mode in the oscilloscope is the edge trigger. This is the trigger mode most people are most likely to use when they first start using an oscilloscope. In addition, the oscilloscope has many other special and even complex trigger modes for responding to specific conditions, and can really make the oscilloscope a powerful measurement tool. These triggers include pulse width trigger, logic trigger, N edge trigger, runt trigger, slope trigger, timeout trigger, video trigger, serial bus trigger, etc.

Vertical scale (volts/division)

By adjusting the vertical scale of the oscilloscope, you can enlarge or reduce the shape of the waveform in the vertical direction. For example, if we set the vertical scale to 1V/grid, and the oscilloscope has 10 grids in the vertical direction, then the entire screen of the oscilloscope can display a waveform of up to 10V. It should be noted that this value is also related to the attenuation ratio of the probe. If we use a 10X probe, but we do not adjust the attenuation ratio of the oscilloscope channel (the default is 1X), then the correct reading will differ by 10 times from the actual displayed reading. Therefore, when using an oscilloscope, you must also pay attention to the attenuation ratio of the probe and adjust the attenuation ratio of the channel to be consistent with the attenuation ratio of the probe.

Input coupling

Input coupling is another simple but commonly overlooked or misunderstood feature in an oscilloscope. It refers to the connection method used to connect an electrical signal from one circuit to another, from the circuit under test to the oscilloscope. You can set the input coupling method to DC, AC, or ground. AC coupling blocks only the DC portion of the signal from passing through, and you will see a waveform centered on zero level on the display. Ground coupling disconnects the input signal from the vertical controls, allowing you to see where zero level is on the display. The DC coupling setting allows all input signals to be displayed, both DC and AC.

Horizontal scale (time/grid)

The horizontal scale function, also called the time base, determines the time that a waveform occupies on the display. Like the vertical scale control described above, the horizontal scale control can also scale the waveform. Compared to the vertical scale control, which controls the vertical direction of the waveform, the horizontal scale controls the horizontal direction of the waveform. If the time base is set to 10ms, each horizontal grid on the display represents 10ms, and the entire screen (assuming a total of 14 grids on the display) is equal to 140ms, which means that the entire waveform displayed is 140ms long. By changing the time base size, you can easily observe longer or shorter time intervals of the input signal.

performance

In terms of the attitude towards the speed of signals, most people generally only care about the frequency of the signal, but not the rise time of the signal. In a standard sine wave, the rise time and frequency are a simple mathematical relationship. The typical formula for determining whether the bandwidth of the oscilloscope is sufficient is 0.35 divided by the rise time. For example, if a pulse with a rise time of 1ns needs to be measured, this means that the minimum bandwidth of the oscilloscope should be around 350MHz. But in reality, Fourier tells us that the actual waveform is a product of a mixture of fundamental waves and higher harmonics. Therefore, the greater the proportion of higher harmonics in the waveform, the shorter its rise time. Compared with the frequency of the signal, the rise time is more representative of the speed of the signal. So don't underestimate low-frequency signals. As long as its rising edge breaks out in an instant, it is enough to cause a series of problems such as signal ringing, reflection, overshoot, etc.

Sample rate (Samples/second) and memory depth are also another important consideration for oscilloscopes. Sample rate refers to the ability of the oscilloscope to collect data points per second. The higher the sample rate, the more realistic and detailed the oscilloscope displays the waveform, and the less likely it is that key information will be lost. If you want to measure a sine wave, a general rule of thumb is that the oscilloscope's sample rate should be at least 2.5 times the highest frequency component of the signal to be measured. If you measure square waves, pulses, and other signal types, the sample rate should be at least 10 times the highest frequency component of the signal to be measured. Memory depth refers to how many sample points the oscilloscope can store on one screen. If the oscilloscope's ability to sample data is sufficient, but its ability to store data is insufficient, then no matter how high the sample rate is, it will be in vain. It's like we want to pour a glass of water. No matter how big the kettle opening is or how fast the water is poured, if our cup is too small, we can't hold much water. Sample rate = memory depth ÷ waveform recording time, this is the relationship between the three. The waveform recording time is the parameter we control. The other two items are generally fixed parameters (oscilloscopes with large storage depths can also adjust the storage depth, but the upper limit is fixed).