TABLE OF CONTENTS Introduction ................................................................................................................................ 4 So what exactly is an oscilloscope? ....................................................................................... 4 Signals ....................................................................................................................................... 7 Frequency measurements .......................................................
Vertical gain ...................................................................................................................... 36 Vertical coupling ............................................................................................................... 36 Trigger controls ................................................................................................................. 36 Delayed timebase ............................................................................................
Introduction This document is a primer on the use and application of analog and digital oscilloscopes (we'll also call them "scopes"). Most of the material is at an introductory level and aimed at helping you understand some of the key features and aspects about oscilloscopes.
We will look at these pictures in more detail, but two observations are: • The peak-to-peak voltage of the waveform can be measured along the vertical axis. It is five main divisions and the vertical gain is set to 200 mV/division (see the yellow arrow), which gives a signal amplitude of 1 volt peak-to-peak. • The horizontal axis is time and the scope is set to 200 µs/division (see the white arrow).
Notation References to sections and figures can be clicked as hyperlinks. The bookmarks contain links to all of the chapters and subsections. The following fonts and colors are used to identify various things: Notation CH1 Coupling Explanation Denotes a control on the front panel of an oscilloscope. Denotes a menu selection in a digital oscilloscope.
Signals With regard to the oscilloscope, the term signal means a voltage that may vary in value as a function of time. One distinction is whether the signal is periodic or not. Periodic means that the signal repeatedly takes on the same set of values over various intervals. The sine wave is one example of a periodic waveform. Let's look at some of its features: Figure 2 Frequency measurements Figure 2 shows two periods of a sine wave.
The sine wave's amplitude in Figure 2 is shown as the distance ‘A’. The mathematical expression for the sine wave, expressing the voltage as a function of time, is: 𝑉 = 𝐴sin(𝜔𝑡) From the graph, you can see that the period of this sine wave is 6.2 ms. This corresponds to a frequency of 1/(6.2 x 10-3 s) or 161 Hz. Then we have 𝜔 = 2𝜋𝑓 = 2𝜋(161𝐻𝑧) = 1011 radian/s. We can also read from the graph that the sine wave's amplitude ‘A’ is 3.2 volts. Thus, the equation for this particular sine wave is: 𝑉 = 3.
While the amplitude A shown in Figure 2 is used in the mathematical expression of the sine wave, it's rarely used in practical measurements. Instead, two other measures are used. We've already discussed the peak-to-peak amplitude and can thus provide the relationship: 𝑉𝑝𝑝 = 2𝐴 (1) where 𝑉𝑝𝑝 is the peak-to-peak amplitude. The amplitude A is sometimes called the zero-to-peak amplitude. Another amplitude measure is RMS, which stands for "root mean square".
Experimentally, it has been established that the heating power of a waveform can be related to a DC situation by the use of RMS measures. Thus, a 1 volt RMS periodic voltage that causes a 1 ampere RMS current through a 1 Ω resistor has an average power dissipated in the resistor of 1 watt. Here, "average" means that the instantaneous power values are averaged over one waveform period or longer. The use of this relationship applies to any shape of waveform, not just sinusoidal waveforms.
Analog oscilloscopes An analog scope is an oscilloscope constructed with analog circuit technology and signals are displayed on a cathode ray tube (CRT), a type of vacuum tube using an electron beam (see the section below on CRTs). Such technology has been evolving since the 1930's when the first commercial oscilloscopes were available. While digital scopes constitute the majority of new oscilloscopes sold, this does not mean there is no demand for analog oscilloscopes.
7 -- This switch chooses the display type. The choices are Main, Mix, Delay, and X-Y. We'll discuss each in more detail below. 8 Position Controls the horizontal position of the trace(s) on the screen. 9 Illum Turns power on to the scope and controls the illumination of the graticule, the scale printed on the CRT. 10 Position Adjusts the vertical position of the trace of channel 1's signal. 11 Position Adjusts the vertical position of the trace of channel 2's signal.
The rear panel has two BNC connectors: • Y-Axis Output Jack - a buffered signal of one of the channels (channel 2 on the 2125A) is available with an output impedance of 50 Ω. It can act as a preamplifier with the same bandwidth of the scope (one use is to amplify a low-level signal for a frequency counter). • Z-Axis Input Jack (also called "External Blanking Input") - a voltage can modulate the intensity of the CRT electron beam.
intensity control adjusts the magnitude of the electron beam current -- the more current, the brighter the spot the beam makes on the screen. The accelerating potential on plate B is not adjustable by the user. The voltages on the plates C and D are the responsibility of the horizontal and vertical deflection circuitry as shown in the following block diagram of a scope: Figure 7 The display circuitry is responsible for generating and adjusting the voltages on the deflection plates.
Horizontal and trigger circuits A block diagram for the horizontal and trigger circuitry is: Figure 9 The horizontal and trigger circuits are responsible for the horizontal movement and positioning of the electron beam. The trigger circuit causes the sweep generator to initiate a sweep of the voltage on the horizontal plates in the CRT. This sweep is a sawtooth-shaped voltage that causes the electron beam to sweep uniformly from the left edge to the right edge of the screen.
The rising edge of the sawtooth sweeps the beam across the screen (this voltage is on the plates in the CRT that deflect the electron beam horizontally). At the right edge of the CRT, the electron beam is turned off and the voltage goes back to what it was at the left edge of the CRT. The holdoff period (control 6 in Figure 5) allows this period to be adjusted. Increasing the holdoff time can make it easier to get stable displays of complex waveforms.
Delayed sweep Some analog oscilloscopes come with a delayed sweep feature that allows a section of the displayed waveform to be magnified in the horizontal direction. This lets you see waveform details while at the same time seeing the whole waveform. The delayed sweep is a second sawtooth generator that is started an adjustable time after the main sweep's sawtooth starts its vertical ramp. After the adjustable delay, the delayed sweep is allowed to control the sweep of the electron beam.
Operation of an analog scope We'll use the B&K 2125A as an example analog scope. It is a popular two-channel 30 MHz scope. A special feature of the scope is that it provides a component tester, which we'll look at in more detail. Over the years there have been a variety of other features provided with analog scopes, such as both digital and analog storage, counters, and digital voltmeters.
Vertical coupling Control 14 in Figure 5 sets the type of coupling to use with the channel 1 signal. It is AC coupling in the up position. This is used to block the DC component of the signal and lets you e.g. observe a small AC voltage riding on top of a DC bias. For example, you might want to look at the AC signal in a transistor amplifier and AC coupling would help you see it in spite of the DC bias on the transistor. However, AC coupling can cause waveform distortion at low frequencies.
If you have an external synchronization signal from the circuit being tested, it can be used to trigger the scope in the external trigger mode. One example of such use would be to view logic signals in a digital circuit -- the scope could be triggered from the clock signal so that the displayed signals would be stable. Trigger holdoff 6 is used to adjust the amount of time delay between subsequent triggers.
two signals. For example, on a 2125A, two 1 MHz sine waves can differ in frequency by 1 part in 107 and the Lissajous pattern will move on the screen, telling you the frequencies are slightly different. The 2125A in component test mode uses the XY display mode. Component test The B&K 2125A oscilloscope provides a component test banana jack. This jack enables a user to quickly test components in-circuit (the circuit must be powered off).
The 2125A's measurement capabilities are: Quantity Useful measurement range If outside range, looks like Resistance 10 Ω to about 10 kΩ < 10 Ω→ short > 10 kΩ → open Capacitance 0.33 µF to 330 µF < 0.33 µF → open > 330 µF → short Inductance 50 mH to 5 H < 50 mH → short > 5 H → open As an example, a 1 H inductor might look like the ellipse in Figure 14 and a 10 H inductor would look like an open circuit. The component tester limits the current through the device.
Digital oscilloscopes We'll use the B&K 2542B-GEN as an example scope. This scope is a two-channel 100 MHz digital scope with a built-in function generator and arbitrary waveform generator. Here's a picture of the front of the scope: Figure 15 The display window of the scope is showing a hyperbolic tangent waveform that is output from the builtin arbitrary waveform generator (connecting cables aren't shown).
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Number Button label F None Soft keys. Their function is shown at the right side of the screen. The button 27 (MENU ON/OFF) can be used to turn the menu on and off. 1 Print When pressed, the displayed waveforms are saved into a file in external memory (e.g., a USB thumb drive). You can choose to either save the waveforms as either a CSV (comma-separated values) or four types of bitmap images (8 bit BMP, 24 bit BMP, GIF, or PNG). 2 CH1 Input male BNC terminal for channel 1's vertical amplifier.
Sets the trigger level to 50% of the amplitude of the waveform being used to trigger the scope. This is useful when NORMAL trigger mode is selected and the scope is not triggering because the trigger level isn't set correctly. 24 50% 25 MENU Turns on trigger menu. 26 LEVEL Adjusts trigger voltage level. Press the knob to set it to 0 volts. 27 MENU ON/OFF For any displayed menu, turns the menu on and off.
A 1 kHz sine wave is displayed on the oscilloscope's screen in the following figure: Figure 17 The various display elements are keyed by letters: Element Description A The cyan-colored symbol indicates that the USB connection is enabled and a flash thumb drive is connected to the scope. The T in the orange polygon indicates the location of the trigger point in the displayed waveform. The 800.
E The yellow 1 indicates the vertical position of 0 volts for channel 1. A similar cyan 2 indicator shows 0 volts for channel 2 when it is displayed. F The displayed waveform for channel 1 shown in yellow. Channel 2 is displayed in cyan. G The small yellow arrow (partially occluded by the letter G) shows that the scope is being triggered on channel 1 and the vertical position with respect to channel 1's zero volts point indicates the trigger level's voltage. In the figure, the trigger voltage is zero.
scope counterparts. Some offer isolated and floating inputs, which are advantages in industrial environments because it means the scope can be used like a digital multimeter (i.e., you don't have to worry that a probe's ground lead will e.g. short out a non-ground potential in a circuit). For budget-minded hobbyists, there are numerous low-cost microcontroller-based projects that let you construct your own oscilloscope.
Sampling Because the idea of sampling is so fundamental to the operation of a digital oscilloscope, let's look at it in more detail. An analogy for sampling is to think of the A/D converter as a camera that takes a picture of the amplitude of the waveform. The "shutter time" is very short and the camera "assumes" that the waveform is constant over the time that the "shutter" is open.
The next plot shows the waveform reconstructions using linear interpolation (i.e., drawing a straight line between each point): Figure 20 The "Adequate sampling" samples reproduce the waveform adequately. With some suitable low pass filtering, this could be an excellent reconstruction of the original waveform. However, the "Inadequate sampling" reconstruction misses important details in the waveform.
In order to accurately reconstruct a signal and avoid aliasing, Nyquist theorem says that the signal must be sampled at least twice as fast as its highest frequency component. This theorem, however, assumes an infinite record length and a continuous signal. Since no oscilloscope offers infinite record length and by definition, glitches are not continuous, sampling at only twice the rate of highest frequency component is usually insufficient.
Since this measured waveform will be a high frequency, thousands of sampling windows can be used to generate thousands of sample points per period -- and thus, the periodic waveform can be accurately displayed by the oscilloscope. Thus, equivalent-time sampling effectively increases the scope's sampling frequency for periodic waveforms. However, be aware that the scope's vertical amplifier bandwidth will likely be the deciding factor on what you're able to see with the scope.
The acquisition time 𝑡𝑎 is determined by the sampling rate, memory depth, and the details of how the scope works. The scope takes a fixed amount of time 𝑡𝑓 to perform the basic calculations needed to display the acquired signal on the screen. If the user has certain features enabled, such as making waveform measurements or placing cursors on the waveform, extra computing time 𝑡𝑣 will be needed to make the requisite calculations (a non-minimum holdoff time can contribute to 𝑡𝑣 also).
The AUTO button One of the biggest time-saving features of a digital oscilloscope is the AUTO button 22. This button tells the oscilloscope to measure the signals on channels 1 and 2 and display them appropriately. The scope manual will tell you what signals the scope will be able to measure automatically (or some experimentation with a function generator will show you). The 254xB scopes require a signal of 50 Hz or greater, a duty cycle of greater than 0.
Vertical gain This adjustment controls the volts per division setting on the vertical scale of the oscilloscope. The settings change in a 1-2-5 sequence. When this control is pressed, a click is felt and the variable adjustment feature is turned on for the vertical gain. The adjustments then decrease or increase in 1% steps of the starting value. The behavior of this adjustment is substantially the same as an analog scope's; see Variable adjustments for more details.
Figure 23 The center of the upper display is the main trace with a narrow black window around the section of the main trace that is expanded in the delayed sweep, the lower trace. The "Z" shows that the delayed timebase is set to a sweep speed of 500 ns/div, or 400 times faster than the main timebase. Thus, we're able to see much more detail in the waveform. The delayed sweep may be able to show you things not easily seen with the main timebase.
The ability to press a button and store a captured waveform or screen image is valuable for documenting investigative and development work. With an analog scope, the user needs to stop and set things up to take a photograph. Contrast this to the digital scope, which just needs a button press and typically less than 2 seconds to capture a screen image. This makes it more likely the user won't be distracted from their work task.
The buttons on the control panel mirror the buttons on the scope and you can operate the scope remotely. This is a powerful tool, as the scope can be across the room or half-way around the world. The web browser should let you save the displayed image of the scope's screen. This is typically done by right clicking the image with the mouse and saving to a file or copying the image to the clipboard.
Recording transient events One of the most important advantages of a digital oscilloscope over an analog oscilloscope is the ability of the digital scope to capture transient (i.e., one-time or infrequent) events and display them. As mentioned above, this is necessarily done with real-time sampling. The steps to acquire a transient waveform are: 1. 2. 3. 4. 5. Set the scope's vertical amplifier(s) to the desired gain and coupling configuration. Set the scope to trigger on the signal you wish to capture.
The following picture demonstrates the ability of the scope to average out random noise: Figure 27 The signal at A was a 200 mV square wave with substantial random noise. At B, the signal has been averaged 16 times and the noise is substantially reduced. At C, 256 waveforms have been averaged and the noise is essentially gone. Averaging can only be used on periodic signals, but since these are often measured, averaging finds frequent use in day-to-day oscilloscope measurements.
Automatic Measurements An advantage of the digital oscilloscope is its ability to make measurements on the displayed waveforms. This provides three benefits: 1. It saves time because the user doesn't have to measure positions on the screen and perform a calculation. 2. It reduces errors, as it's not unusual for a user to do the requisite calculations in their head and make a mistake. 3. The measurements can typically be made at a higher precision than the user can get from the screen.
Figure 30 The three displayed measurements at the bottom of the screen are the RMS voltage of the channel 1 signal (yellow) and its frequency. The frequency of the channel 2 signal is displayed in blue. The two frequency measurements show that these sine waves differ by 12 Hz in frequency. This would be hard to discern at this level by making measurements on the screen. These measurements were taken with a B&K 2542B scope.
Figure 31 The white trace is the reference waveform that was saved to internal memory (a reference waveform can also be saved to a flash drive). The yellow trace is the signal on channel 1 of the scope. The easiest way to compare these two signals is to adjust the vertical position of channel 1's signal. In the situation shown in the picture, the two signals are exact matches. A signal amplitude change of 1% is discernible and a 2% change is easy to see.
waveform w2. Each recorded waveform is called a frame. The B&K 2542B scope lets you set Δ t to values between 1 ms and 1000 s and record from 1 to 1000 frames. You can save the recorded frames to internal or external storage (such as a thumb drive). This lets you review the recorded frames at a later time. You can turn the scope's general-purpose knob and "page" through the frames looking for unusual behavior.
The scope's digital filter was set to be a low-pass filter with a cutoff frequency of 1.2 kHz. This effectively removed the higher signal component and lets us measure the amplitude of the fundamental. The generator was set to 1 Vpp and you can see that is approximately the peak-to-peak amplitude in waveform A. You can estimate that the peak-to-peak amplitude of the fundamental in A is about 400 mV and this is confirmed in trace B.
WaveXpress® Since a digital oscilloscope can have an interface to allow a computer to communicate with it, a program running on a computer can be used to upload and download information to an oscilloscope. B&K Precision provides the WaveXpress® program free of charge for this purpose (see http://www.bkprecision.com/wavexpress.html).
Probes Probes are the most common methods for connecting the oscilloscope to the circuit of interest. There are two basic types of probes available,active and passive. An active probe contains active circuitry (i.e.., semiconductors and perhaps an external power supply). These probes can give the highest performance, but can be substantially more expensive than passive probes.
1M𝛺 = 0.1 (9 + 1)M𝛺 This is why the probe is called a 10X probe as it attenuates the signal by 10 times. You'll also see them called X10 probes. The cable's distributed capacitance and the scope's input capacitance will shunt the signal around Rs at higher frequencies -- this leads to the basic fact that a passive probe's impedance at its rated bandwidth can be more than four orders of magnitude smaller than its DC resistance.
𝑡𝑝 = �𝑡 2 − 𝑡𝑠2 You can calculate the scope's rise time 𝑡𝑠 in ns from: where B is the scope's bandwidth in MHz. 𝑡𝑠 = 350 𝐵 Probe compensation Probe compensation is the process of matching the probe's electrical characteristics to the scope's. The result is that signals viewed by the scope using the probe will be accurately depicted (excluding the roll-off due to the probe's bandwidth). Probe compensation is usually done with a square wave signal provided on the scope's front panel.
A passive probe and its accessories Let's look at a typical passive probe. The following picture shows a B&K Precision PR37AG 150 MHz 1X/10X switchable probe and its 15 cm ground clip that ends in an alligator clip (the scale is graduated in mm): Figure 37 The orange switch has positions X1, X10, and REF. The REF position grounds the center conductor and disconnects the input, allowing you to see the position of 0 volts on the oscilloscope's screen.
The device at a is called a bayonet-mount spring tip. It is used on high frequency circuits because it eliminates the long ground lead inductance and reduces ringing. The tip at b allows the user to probe IC pins. The tip at c is for general-purpose probing and insulates the tested circuit from the grounded band of metal. The adapter at d allows the probe to be plugged into a BNC female connector. This makes for a convenient connection to test equipment and to BNC test ports on circuits.
Position 1X: Attenuation ratio 1X (1:1) Bandwidth DC to 6 MHz (-3 dB) Rise time 58 ns Input resistance 1 MΏ (oscilloscope input resistance) Input capacitance 56 pF (plus oscilloscope capacitance) Max. input voltage 300 V CAT I, 150 V CAT II (DC + peak AC) derated with frequency (see figure below) Pollution Degree 2 Max. operating temperature 0 °C to 50 °C Humidity 85% RH or less (at 35 °C) Cable length 1.2 m (48") Max.
Floating a scope Some people choose to "float" a scope so they can make a differential measurement with a probe. This is done to remove the connection between the scope's chassis (and the outside of the BNC jacks) to the power line ground. Then the user reasons that they can connect the probe and the probe's ground lead into a circuit to e.g. measure the voltage across a resistor (a common technique to look at the current in a circuit).
Good measurement practices These are provided as guidelines of good practice, but may not be true in all situations. 1. Divide the bandwidth by 10 to get a rough idea of the fundamental frequency of an arbitrary periodic signal that your scope will be able to reasonably reproduce. 2. Time measurements are generally relative with oscilloscopes -- thus, you usually subtract two times measured on the screen to get a time difference.
Oscilloscope safety Remember that your safety (and often the safety of others working near you) is ultimately your responsibility. Take this responsibility seriously and be methodical about it. Don't engage in horseplay. Use checklists to remind you of things that need to be done. The cost and effort of safety training and practicing its rules will seem like a trivial expense compared to the human cost of an accident or a death after the fact. 1.
Glossary Alternating current. It refers to a voltage or current that is periodically changing over time. It can also refer to the type of electrical coupling to a scope's AC vertical amplifier or trigger circuitry. AC coupling means that the DC component of a waveform is blocked. The potential used to accelerate the electrons in a scope's CRT.
bandwidth blanking blind time BNC CAT I, CAT II, CAT III, CAT IV channel chopped sweep CMRR compensation component test coupling cross talk In the context of oscilloscopes, this is the upper frequency rating of the oscilloscope's vertical amplifier(s) (the scope's lower frequency rating is 0 Hz, as it can measure DC voltages).
CRT cursor DC DDS dead time decibel delayed time base digitize division DSO dual trace duty cycle envelope equivalent time sampling extrinsic noise fall time focus frequency gain Cathode ray tube. It is the vacuum tube with a thermionic emitter and electrostatic deflection plates (some may also contain deflection coils) that accelerates electrons that impact on the phosphor screen, producing light. A CRT is used with virtually all analog oscilloscopes.
glitch graticule grid ground intensity interpolation intrinsic noise linearity Lissajous figure loading main time base mixed signal oscilloscope noise oversampling peak detection peak-to-peak period persistence phase An unexpected signal or portion of a repetitive waveform that is unlike the other parts of the waveform. Glitches tend to be of short duration compared to the signal of interest. Other terms that can indicate unexpected signals are spike, runt pulse, or ringing.
phosphor post-trigger post-trigger data pre-trigger pre-trigger data probe pulse pulse period pulse train pulse width ramp raster real time sampling record length rise time RMS sampling sampling rate sampling scope A chemical used to coat the inside of a CRT. When struck by fast-moving electrons, the orbital electrons of the phosphor are excited to higher energy levels using the kinetic energy of the incoming electrons.
screen single sweep single-shot slope sweep sweep magnifier sweep speed TCO thermionic emission time base trace transient trigger trigger holdoff trigger level trigger mode trigger slope The visual display area of an oscilloscope. It can be a CRT (a phosphorcoated electron beam tube), an LCD (liquid crystal display), or an LED (light emitting diode) display. A mode of operation of a digital scope where the scope is "armed" to wait for a trigger event.
TV sync unarmed vertical attenuator vertical gain vertical sensitivity video sync WaveXpress® x-axis XY display y-axis z-axis See video sync. In referring to an oscilloscope's trigger section, an unarmed trigger is when the trigger is not armed. The scope will not trigger in the unarmed state. The attenuator used on a vertical channel of a scope. The gain setting of the vertical amplifier. Usually given in volts per graticule division.
References WaveXpress® is a registered trademark of B&K Precision Corporation. To contact B&K Precision, please visit http://www.bkprecision.com/ or contact us at: B&K Precision Corporation 22820 Savi Ranch Parkway Yorba Linda, CA 92887-4610, USA 714-921-9095 714-921-6422 (fax) 800-462-9832 (US toll free) Allen-Bradley, System Design for Control of Electrical Noise, Publication GMC-RM001A-EN-P, July 2001. Williams, J., Linear Technology, High Speed Amplifier Techniques, Application note AN-47, August 1991.