If there’s one tool that anyone responsible for analyzing or repairing electrical circuits would not be without it would be a multimeter. However, when more sophisticated measurements need to be made, an oscilloscope is often employed. These devices allow more of the characteristics of an electrical signal to be measured.
Like most instrumentation, the complexity of oscilloscopes has advanced with the advent of digital technology. Older models, however, provide a good framework for understanding the basics of how they work and what information can be gleaned from hooking an oscilloscope up to an electrical circuit.
To understand the basics of an oscilloscope it’s useful to discuss a simple model. The model for example purposes will be an analogue model. Be aware that, with digital models, some of the subjects discussed here are actually much simpler than they are on an analogue model, but because information is displayed so readily using digital oscilloscopes as examples doesn’t serve as well as analogue scopes do in explaining the function of an oscilloscope.
An oscilloscope is typically attached to a circuit via a hook and wire lead. In most designs, the positive and negative terminals are contained in the same probe, making these devices quite convenient to use.
The ground is attached to a ground point on the circuit and the hook lead is attached to the point in the circuit where the electrical signal needs to be measured. Once the probes are in place, the display will show the waveform.
On an older design, and oscilloscope is typically provided with a cathode ray tube display. These have the classic bluish green tint to them and look somewhat like an older style television screen.
On the screen, lines are drawn forming a grid. The grid serves as indicator lines for a graph. The value for each of the intervals on the graph can be changed by adjusting the controls on the oscilloscope.
Other controls on the oscilloscope allow the wave to be enlarged or reduced in size – only on the display, not electrically – allow the wave to be advanced or backed up so that a certain portion of the wave can be closely analysed and the interval of time shown on the graph can be changed, allowing a single cycle of the wave to be isolated or many different cycles to be viewed at the same time.
The line displayed on the oscilloscope screen is referred to as a trace. It will display as a sine, sawtooth, triangle, or square wave, showing the voltage behaviour in the circuit over time. By enlarging the value of each of the increments on the grid, the display of the wave can be shrunk, allowing an easier overview. By decreasing the value of the intervals, voltages can be measured very precisely.
On a typical oscilloscope display, the central X and Y axes are broken into very small hashes, allowing for particularly precise measurements. To get precise measurements, whichever part of the wave needs to be analysed is centred on the more narrowly graduated axes, allowing those measurements to be made.
Where the basics of the graph are concerned, the divisions of the graph that traverse the screen vertically represent voltage increments. The divisions of the graph that traverse the screen horizontally represent time increments. In most cases, the middle lines represent zero voltage. Lines that drop below the central X axis represent negative voltages and lines that exist above the central X access represent positive voltages. This makes these devices particularly convenient where measuring AC power is concerned.
Time increments are generally very flexible on these devices, as well. Most of them will allow the user to measure time intervals as small as .55 µs, with some devices allowing even smaller intervals of time to be measured.
It is possible to slow down the device’s measuring cycle enough to observe the sweep. The sweep is the term used to refer to the device drawing the line across the screen, which traverses from left to right. In most cases, however, when measurements are being taken, the wave is displayed in a static form on the screen, allowing the operator to zoom in and out as required, to expand and compress the timeframe being shown and so forth.
The different forms of waveform measurable with oscilloscopes allow different electrical properties to be measured. For example, the classic sine wave, which traverses from positive to negative voltages over time, indicates an alternating current. A square wave, which goes from zero to a peak voltage and straight down to zero once more represents a circuit that is simply turning on and off, or possibly a component in a digital device. Triangle waves are similar to square waves. Sawtooth waves represent a combination of a square wave and a triangle wave.
Inside an Oscilloscope
Oscilloscopes are complex devices, but the most important components in a CRT model are the electromagnets on either side of the screen. Electrons are shot at the glass on the screen, which is treated with a substance that causes the screen to glow when the electrons hit it. Within the device, the electromagnets on either side of the screen pull the electrons from one side to another, creating the trace.
Digital oscilloscopes, just as is the case with digital multimeters, have largely replaced analogue oscilloscopes. Digital oscilloscopes provide expanded and more convenient measurement capabilities and, importantly, are much more portable than analogue designs. Because of the CRT screens and the other bulky analogue hardware, analogue oscilloscopes are typically equipment found permanently affixed to lab benches. Digital oscilloscopes, on the other hand, are often quite portable.
A digital oscilloscope does not directly display the electrical signal it is measuring, as does an analogue device. In a digital oscilloscope, the analogue signal is converted to a digital one through an analogue to digital converter and that data is displayed on the screen.
Like other digital devices, digital oscilloscopes take a sample of a signal and display that information to the user. In some instances, developers may require an analogue oscilloscope, simply because they are directly looking at the signal, rather than looking at a sample of it. However, in many instances, digital oscilloscopes provide a greater level of convenience, a better price point and measurements that are more than adequate for most applications.
Digital oscilloscopes have some advantages where data is concerned. Some of these devices come outfitted with USB connections. These can be used to directly dump data into a computer for analysis. While digital oscilloscopes lack the ability to directly examine electrical signals, they do have the ability to store data gathered and that makes them enormously useful in many applications.
On some modern digital oscilloscopes, information that would have to be gleaned by reading the graph on an older model is displayed outright. The voltage of the signal, for instance, may simply be displayed on the screen, allowing the user the option of looking at the graph for certain information and getting precise readings of the values in others. This can make it much faster to take readings with the devices.
Digital oscilloscopes are also capable of carrying out math operations. This means that it is no longer down to the operator to do this manually and, of course, that reduces the chances for error in measurement. Between the portability, ease-of-use, and cost-effectiveness of digital oscilloscopes, they are very common equipment in industrial and electrical engineering settings. Some situations, however, may require the benefits of both digital and analog oscilloscopes, which is why analog oscilloscopes are still widely available and commonly employed in engineering applications.
Best of Both Worlds
There are oscilloscope designs available that incorporate both analog and digital inputs. These devices are manufactured by companies such as Tektronix, LeCroy, and Pico Technology. Typically, they come with two or four analog channels along with many more digital channels, allowing for a great deal of flexibility. Some of these designs are benchtop equipment and, in the most advanced cases, they are expensive, high precision devices that are used in the most demanding engineering applications.
Where the digital portion of these oscilloscopes is concerned, the important specifications are the same as they would be for a purely digital oscilloscope. The band will determine the frequency of the signals that can be measured with the device. The sampling rate will determine how often the device is able to sample electrical properties.
Utilizing one of these combination devices, someone working on a circuit can closely examine the electrical properties of a signal as it is passing through the circuit, just as they would be able to do on any other analog oscilloscope. The digital channels allow the user to capture data, analyze it in whatever program they wish, and take advantage of everything a digital oscilloscope offers, as well. For many industrial purposes, research and development purposes, and engineering purposes, a combination device is an excellent solution compared to buying separate digital and analog oscilloscopes.
However, in situations where measurements need to be taken in the field, a digital oscilloscope may be the first choice, with an analog oscilloscope only being used under certain circumstances on a workbench. Whether they are purchased separately or as one device, digital and analog oscilloscopes both offer distinct advantages and, though they may essentially be the same type of tool, each of them has a specific place in today’s electrical engineering world.
Oscilloscopes typically entail accessories. For example, the large, workbench-style oscilloscopes generally require mounting devices to ensure that they are safe and easy to manipulate. Various types of probes can be purchased to use with the oscilloscopes, allowing them to be used on different types of circuitry and to consistently take precise and reliable measurements. Oscilloscopes that are portable will typically operate on a rechargeable battery pack and, as these are used up, they can be changed out and replaced, ensuring reliable performance.
Software is a significant and rather new concern for purchasing oscilloscopes. Oscilloscope software allows data captured with a digital oscilloscope to be analyzed in very sophisticated ways. Importantly, it allows the engineer to make these measurements and analyses without having to do the math themselves. This can save a great deal of time and improve overall accuracy. Generally, oscilloscope software is designed to work with specific models of the oscilloscope.
For portable designs, cases and spare probes are generally advisable, which helped to prepare for the sometimes unpredictable conditions that someone using one of these devices will encounter on a factory floor or in other environments outside the workshop. There are many different accessories available for these measurement tools, given that they are so fundamental to electrical troubleshooting and engineering.
Why They’re So Useful
The most basic measurement that in oscilloscope makes, the behaviour of voltage over time, is really just one of the uses of these devices. In analysing circuits, oscilloscopes can be used to measure phase shift, they can use to be measure capacitance and induction, they can be used to look for irregularities in electric signals, noise in electric signals and many other variables that need to be measured by engineers.
Compared to a digital multimeter, an oscilloscope is a much more dynamic measuring device. A digital multimeter gives one number. For example, hooking up a digital multimeter to a circuit that has 5 V total will yield a meter reading of 5 V, or close to it. On the oscilloscope, the engineer cannot only get information on the amount of voltage going through the circuit, they can also get information about how that voltage might change – predictably or unpredictably – over the course of time.
Taking a closer look at the sine wave, engineers can get very precise with their measurements, allowing them to troubleshoot circuits and design circuits with a great deal of precision. In oscilloscope is something that no electronic workshop is complete without. From circuit design to troubleshooting to repair, they are indispensable tools and, because digital oscilloscopes offer an entirely new option, they have become more flexible and powerful than ever.