Oscilloscopes Jacob Centner PHYS 3650

What is an Oscilloscope? [1] An oscilloscope measures voltage and displays it as voltage over time (most frequently). The display’s amplitude and timescale are adjustable. Repetitive signals can be displayed as a standing wave by proper adjustment. They are used in physical and biological sciences, medicine, telecom, engineering, many industries. As was mentioned in class when we talked about these, not incredibly accurate, but it is graphical, which makes it powerful.

Comparison to Voltmeter [1] Extraordinarily quick response to voltage change (nanoseconds) Limited accuracy Display voltage as function of time A typically voltmeter, as we covered somewhat in class, reacts slowly to a change in voltage and is best used to display steady-state voltage with good accuracy. An oscilloscope, on the other hand, can react on the nanosecond scale to changes in voltage. However, its measurements are more approximate. Its great value lies in its capability of graphically displaying the voltage as a function of time, allowing the analysis of complex waveforms.

Types of Oscilloscope [1] Cathode-Ray (Analog) Digital The two types of oscilloscope are shown here. The analog version uses a CRT for its display; the digital version converts the analog voltage input to digital data, which is then handled by an embedded computer. While one may assume that the digital oscilloscope is “better” than an analog one since it is more recent technology, there are in fact numerous advantages and disadvantages to using each kind, which I will cover later. There are also mixed signal oscilloscopes, which attempt to combine the two by allowing both analog and digital input channels.

Oscilloscope Engineering [1] Vertical amplifier Trigger Circuit Time Base Display There are four main components underpinning every oscilloscope. These components and their principles of operation apply both to analog and digital o-scopes, with minor variations. The vertical amplifier allows the positioning and scaling of the input voltage to be readable on the display. It is directly controlled by the user. The trigger starts the display at the same point on the input signal every time the display is refreshed. This creates a “standing wave” to be viewed. The time base or horizontal system causes the input voltage to be display as a function of time. It permits the adjustment of the timescale shown and the position of the waveform on the screen horizontally (the delay).

Principles of operation, Advantages, and Shortcomings [1] Analog Oscilloscopes The analog oscilloscope uses a CRT for its display. An electron beam is fired toward a phosphor-coated screen and is deflected using a magnetic field. The intensity of the light is positively correlated with the number of electrons that hit the phosphor on that part of the screen. This is not an entirely continuous system: while the screen is being refreshed, the o-scope is blind. The display is frequently refreshed, and each refresh is very quick, allowing it to display rapid voltage changes. The knobs adjusting the vertical amplifier and time base generator (controlling vertical and horizontal positioning and scale) react very quickly. Operation is relatively simple, with the aforementioned knobs providing direct control of the device. The CRT is the source of the most shortcomings for analog oscilloscopes. Since the display must be constantly refreshed, there is no way to store the waveform. That means that input signals which fail to repeat with sufficient frequency will fail to be displayed. If the frequency of the input drops below ~100Hz, the display will flicker, and lower frequency signals will appear as only a flash of light. Another shortcoming is timing inaccuracy, which in some cases can result in up to a 30% variation in measurement. Obviously the electron beam can be deflected by strong fields nearby, so check your environment before using one. Principles of operation, Advantages, and Shortcomings [1]

Digital Oscilloscopes In a digital o-scope, the input signal is converted into digital data, which is then handled by an embedded processor. The processor can apply error corrections to the input automatically, compensating for calibration errors in the vertical system. The data can then be either stored or displayed. Compared to an analog system, digital o-scopes can achieve better accuracy due to the error correction performed by the embedded processor. Timing accuracy in a digital o-scope is an order of magnitude better than an analog system. One of the greatest advantages is that input data can be stored, either for output to a computer, or for viewing very short-time or non-repeating signals. On the other hand, digital o-scopes are more complex to operate and suffer from poorer display performance. In particular, the primary disadvantage of these devices is that the display has a significantly longer dead- or blind-time and have lower (typically half) the resolution of analog o-scope displays. The speed at which the display refreshes is limited by the speed of the processor; scopes utilizing a single CPU must wait for all its operations to complete before they can display the waveform. This introduces input delay when adjusting the front-panel controls. Now, these shortcomings may be somewhat attenuated by recent advancements in processor and display technology. The resource from which I drew these shortcomings is a little dated, and technology has progressed in the meantime. Very new digital oscilloscopes may have rectified these faults. Differences from Analog, Advantages, and Shortcomings [1]

Comparison of Features [1] The advantages and disadvantages are nicely summarized by this table. Aliasing refers to the misreproduction of a signal caused by undersampling, introducing distortion.

Terminology Example Waveform [1, 2] There are a number of terms that must be understood before I continue. First, on the image here you see overshoot and preshoot labeled. I won’t go into the math for the sake of time, but in order to construct a signal, oscilloscopes use a fourier series and sum over all the frequency components of the signal. Overshoot and preshoot, or undershoot, occur because the discontinuity of a square wave causes some issues with Fourier series. Next, when an oscilloscope measures voltage, there is a non-zero delay in the vertical system as it “adjusts” to the new voltage. This is called the rise. Divisions or “divs” on an o-scope refer to the scaling – in this image you can see each div clearly marked. The vertical scale is adjusted to so many volts/div, this one’s on 100mV. Horizontal is in nanoseconds/div or whatever timescale is chosen. Full-scale is a term that refers to the entire axis. On a 1 Volt/div setting, full scale on this o-scope would be 8 Volts as there are 8 divs. Some o-scopes have 10 divs on the vertical axis. This one has 8 vertical, 10 horizontal. Accuracy specifications are always expressed in terms of full scale, as I’ll cover later. Terminology Example Waveform [1, 2]

Measuring Voltage with Oscilloscopes [1] Analog Digital Use measurement lines printed on the screen Entirely done by sight Additional measurements require more skill by the operator Measurement lines can be used for quick calibration Embedded processor performs automated measurements and calculations When measuring with an o-scope, an important difference between analog and digital devices is their method of measurement. Analog o-scopes rely entirely on human measurement, while digital ones typically offer more reliable calculated measurements.

Using an Oscilloscope [2, 3] Control knobs Channels Trigger AC/DC Offset There are a number of basic controls on an oscilloscope to be familiar with. Knobs allow for scaling the volts/div and time/div settings to scale the vertical and horizontal. The trigger is configurable and its adjustment allows repetitive waveforms to be shown in a still, standing wave. Note that there are AC and DC current modes for each channel; remember to use the appropriate one. Offset control allows for centering the waveform for better display and reduced error.

Error to Consider and Understanding Specifications [1] Analog Digital Vertical Calibration for high frequency Accuracy specifications limited to center 8 horizontal divs Generally less accurate than digital Gain error Digitizing Resolution error Offset/Position error Vertical Single-Cursor vs Dual-Cursor Similar scheme for horizontal Manufacturers of o-scopes post a number of specifications about their designs. I don’t have time to go over them all but here are some highlights from each type. Analog o-scopes require careful calibration when measuring very high frequencies because their frequency response is a Gaussian distribution in frequency, not flat. Error can quickly be as high as 30%. Other instruments are typically used to measure at frequencies this high. Digital o-scopes have the same problem, but it is accounted for by the processor typically. The horizontal accuracy specifications for analog scopes are commonly only applicable to the center 8 divs. Readings in the outermost divs may be more error-prone than specifications lead on. Digital scopes suffer from a number of types of error that is frequently combined into either single-cursor or dual-cursor error ratings. Vertical gain error, digitizing resolution error, and offset error are all vertical accuracy specifications that are summed into single-cursor accuracy, which is used when a single cursor is manually controlled and positioned in order to read a voltage. When two cursors are used to measure voltage, the offset error is cancelled out. There are other types of specifications for the horizontal, like trigger placement resolution. They are all summed into “cursor accuracy” as well.

Pro Tip for Minimizing Error Use the tallest and widest vertical and horizontal display of the signal (maximize signal size on display) [1] Vertical accuracy is a full-scale specification, meaning that at 1V/div, full scale is ~8V, and +/-3% accuracy is +/- 0.24V. That is a 24% error for a 1V signal. At 250mV/div, full scale is ~2V, so error is correspondingly quartered to 6%. Same applies for horizontal Tallest and widest vertical and horizontal: use the highest volts/div setting you can because the accuracy of measurement degrades considerably as this setting is reduced.

Questions?

Bibliography [1] Jerry Murphy, ‘The Measurement, Instrumentation, and Sensors Handbook: Oscilloscopes,’ 1999, p. 37-22 – 37-41. [2] Roger Traylor, ‘Making Oscilloscope Measurements,’ accessed 13 February 2019, http://web.engr.oregonstate.edu/~traylor/ece391/ECE406/391_hh/hh1.pdf [3] ‘Oscilloscope Basics,’ accessed 12 February 2019, https://courses.cs.washington.edu/courses/csep567/05au/tools/oscopefordummies/Os copeBasics.html