Electronic technology permeates our lives and as electronic technology has advanced, the speeds at which these devices operate have accelerated. Today, most devices use high-speed digital technologies. Engineers need the ability to accurately design and test the components in their high-speed digital devices. The instrumentation engineers use to design and test their components must be particularly well-suited to deal with high speeds and high frequencies. An oscilloscope is an example of just such an instrument.
The main purpose of an oscilloscope is to display electronic signals. By viewing signals displayed on an oscilloscope you can determine whether a component of an electronic system is behaving properly. So, to understand how an oscilloscope operates, it is important to understand basic signal theory.
Electronic signals are waves or pulses. Basic properties of waves include:
Two main definitions for amplitude are commonly used in engineering applications. The first is often referred to as the peak amplitude and is defined as the magnitude of the maximum displacement of a disturbance. The second is called the root-mean-square (RMS) amplitude. To calculate the RMS voltage of a waveform, square the waveform, find its average voltage and take the square root. For a sine wave, the RMS amplitude is equal to 0,707 times the peak amplitude (Fig. 1).
Phase shift refers to the amount of horizontal translation between two otherwise identical waves. It is measured in degrees or radians. For a sine wave, one cycle is represented by 360 degrees. Therefore, if two sine waves differ by half of a cycle, their relative phase shift is 180 degrees.
The period of a wave is simply the amount of time it takes for a wave to repeat itself. It is measured in units of seconds.
Every periodic wave has a frequency. The frequency is simply the number of times a wave repeats itself within one second (if you are working in units of Hertz). The frequency is also the reciprocal of the period.
A waveform is the shape or representation of a wave. Waveforms can provide you with a great deal of information about your signal. For example, it can tell you if the voltage changes suddenly, varies linearly, or remains constant. There are many standard waveforms, but this section will cover the ones you will encounter most frequently.
Sine waves are typically associated with alternating current (AC) sources such as an electrical outlet in your house. A sine wave does not always have a constant peak amplitude. If the peak amplitude continually decreases as time progresses, we call the waveform a damped sine wave (Fig. 2).
A square waveform periodically jumps between two different values such that the lengths of the high and low segments are equivalent. A rectangular waveform differs in that the lengths of the high and low segments are not equal (Fig. 3).
In a triangular wave, the voltage varies linearly with time. The edges are called ramps because the waveform is either ramping up or ramping down to certain voltages. A sawtooth wave looks similar in that either the front or back edge has a linear voltage response with time. However, the opposite edge has an almost immediate drop (Fig. 4).
A pulse is a sudden single disturbance in an otherwise constant voltage (Fig. 6). Imagine flipping the switch to turn the lights on in a room and then quickly turning them off. A series of pulses is called a pulse train. To continue our analogy, this would be like quickly turning the lights on and off over and over again. Pulses are the common waveform of glitches or errors in your signal. A pulse might also be the waveform if the signal is carrying a single piece of information.
Waves can also be mixtures of the above waveforms. They do not necessarily need to be periodic and can take on very complex wave shapes.
Analogue versus digital signals
Analogue signals are able to take on any value within some range. It is useful to think of an analogue clock. The clock hands spin around the clock face every twelve hours. During this time, the clock hands move continuously. There are no jumps or discreteness in the reading. Now, compare this to a digital clock. A digital clock simply tells you the hour and the minute. It is, therefore, discretised into minute intervals. One second it might be 11:54 and then it jumps to 11:55 suddenly. Digital signals are likewise discrete and quantised. Typically, discrete signals have two possible values (high or low, 1 or 0, etc.). The signals, therefore, jump back and forth between these two possibilities.
What is an oscilloscope and why do you need one?
An oscilloscope is a measurement and testing instrument used to display a certain variable as a function of another. For example, it can plot on its display a graph of voltage (y-axis) versus time (x-axis). The main purpose of an oscilloscope is to give an accurate visual representation of electrical signals. For this reason, signal integrity is very important. Signal integrity refers to the oscilloscope’s ability to reconstruct the waveform so that it is an accurate representation of the original signal. An oscilloscope with low signal integrity is useless because it is pointless to perform a test when the waveform on the oscilloscope does not have the same shape or characteristics as the true signal. It is, however, important to remember that the waveform on an oscilloscope will never be an exact representation of the true signal, no matter how good the oscilloscope is. This is because when you connect an oscilloscope to a circuit, the oscilloscope becomes part of the circuit. In other words, there are some loading effects. Instrument makers strive to minimise loading effects, but they always exist to some degree.
In general, modern digitising oscilloscopes look similar to the one seen in Fig. 7. However, there are a wide variety of oscilloscope types, and yours may look very different. Despite this, there are some basic features that most oscilloscopes have. The front panel of most oscilloscopes can be divided into several basic sections: the channel inputs, the display, the horizontal controls, the vertical controls, and the trigger controls. If your oscilloscope does not have a Microsoft Windows-based operating system, it will probably have a set of softkeys to control on-screen menus.
An oscilloscope can do more than plot voltage versus time. An oscilloscope has multiple inputs, called channels, and each one of these acts independently. Therefore, you could connect channel 1 to a certain device and channel 2 to another. The oscilloscope could then plot the voltage measured by channel 1 versus the voltage measured by channel 2. This mode is called the XY-mode of an oscilloscope. It is useful when graphing I-V plots or Lissajous patterns where the shape of these patterns tells you the phase difference and the frequency ratio between the two signals. Figure 8 shows examples of Lissajous patterns and the phase difference/frequency ratio they represent.
Types of oscilloscopes
The first oscilloscopes were analogue oscilloscopes, which use cathode-ray tubes to display a waveform. Photoluminescent phosphor on the screen illuminates when an electron hits it, and as successive bits of phosphor light up, you can see a representation of the signal. A trigger is needed to make the displayed waveform look stable. When one whole trace of the display is completed, the oscilloscope waits until a specific event occurs (for example, a rising edge that crosses a certain voltage) and then starts the trace again. An untriggered display is unusable because the waveform is not shown as a stable waveform on the display (this is true for DSO and MSO oscilloscopes, which will be discussed below, as well.)
Analogue oscilloscopes are useful because the illuminated phosphor does not disappear immediately. You can see several traces of the oscilloscope overlapping each other, which allows you to see glitches or irregularities in the signal. Since the display of the waveform occurs when an electron strikes the screen, the intensity of the displayed signal correlates to the intensity of the actual signal. This makes the display act as a three-dimensional plot (in other words, x-axis is time, y-axis is voltage, and z-axis is intensity).
The downside of an analogue oscilloscope is that it cannot “freeze” the display and keep the waveform for an extended period of time. Once the phosphorus substance deluminates, that part of the signal is lost. Also, you cannot perform measurements on the waveform automatically. Instead you have to make measurements usually using the grid on the display. Analogue oscilloscopes are also very limited in the types of signals they can display because there is an upper limit to how fast the horizontal and vertical sweeping of the electron beam can occur. While analogue oscilloscopes are still used by many people today, they are not sold very often. Instead, digital oscilloscopes are the modern tool of choice.
Digital storage oscilloscopes (DSOs)
Digital storage oscilloscopes (often referred to as DSOs) were invented to remedy many of the negative aspects of analogue oscilloscopes. DSOs input a signal and then digitise it through the use of an analogue-to-digital converter. Figure 9 shows an example of one DSO architecture used by Keysight Technologies’ digital oscilloscopes.
The attenuator scales the waveform. The vertical amplifier provides additional scaling while passing the waveform to the analogue-to-digital converter (ADC). The ADC samples and digitises the incoming signal. It then stores this data in memory. The trigger looks for trigger events while the time-base adjusts the time display for the oscilloscope. The microprocessor system performs any additional postprocessing you have specified before the signal is finally displayed on the oscilloscope.
Having the data in digital form enables the oscilloscope to perform a variety of measurements on the waveform. Signals can also be stored indefinitely in memory. The data can be printed or transferred to a computer. In fact, software now allows you to control and monitor your oscilloscope from a computer using a virtual front panel.
Mixed signal oscilloscopes (MSOs)
In a DSO, the input signal is analogue and the digital-to-analogue converter digitises it. However, as digital electronic technology expanded, it became increasingly necessary to monitor analogue and digital signals simultaneously. As a result, oscilloscope vendors began producing mixed signal oscilloscopes that can trigger on and display both analogue and digital signals. Typically there are a small number of analogue channels (2 or 4) and a larger number of digital channels.
Mixed signal oscilloscopes have the advantage of being able to trigger on a combination of analogue and digital signals and display them all, correlated on the same time base.
As its name implies, a portable oscilloscope is one that is small enough to carry around. If you need to move your oscilloscope around to many locations or from bench to bench in your lab, then a portable oscilloscope may be perfect for you. The advantages of portable oscilloscopes are that they are lightweight and portable, they turn on and off quickly, and they are easy to use. However, they tend to not have as much performance power as larger oscilloscopes.
Economy oscilloscopes are reasonably priced, but they do not have as much performance capability as high-performance oscilloscopes. These oscilloscopes are typically found in university laboratories. The main advantage of these oscilloscopes is their low price. For a relatively modest amount of money, you get a very useful oscilloscope.
High-performance oscilloscopes provide the best performance capabilities available. They are used by people who require high bandwidth, fast sampling and update rates, large memory depth, and a vast array of measurement capabilities. The main advantages of a high-performance oscilloscope are that the scope enables you to properly analyse a wide range of signals, and provides many applications and tools that make analysing current technology simpler and faster. The main disadvantages of high-performance oscilloscopes are their price and size.
Oscilloscopes are a powerful tool in the technological world we currently live. They are used in a wide range of fields and offer many advantages over other measurement and testing devices. After reading this article, you should have a good feel for oscilloscope fundamentals.
Contact Niven Thamanna, Concilium Technologies, Tel 012 678-9200, Niven.Thamanna@concilium.co.za