10 Easy Ways to Troubleshoot with an Oscilloscope

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These 10 simple exercises — beginning with the simplest and moving to the more difficult — will help sharpen your skills in the field.

Today’s oscilloscopes have far more functionality than their old cathode-ray tube (CRT) analog counterparts. At first glance, the proliferation of all of those knobs and buttons may seem overwhelming, but the on-screen menus and on-line documentation capabilities of today’s instruments enable you to display waveforms and measure/save inputs with ease.

Most makes and models currently available are similar in price and features. Although the terminology and layout of controls may differ from unit to unit, the basic methods for getting signals to display are quite similar.

This series of 10 oscilloscope demonstrations — beginning with the simplest and moving to the more difficult — is intended to give you an idea of what these amazing instruments are capable of doing in the new world of advanced electronics and to help increase your troubleshooting proficiency along the way.

Let’s begin with a simple DC hookup. To get started, plug a probe into one of the analog channel inputs, and press the power button. Like a computer, the instrument will take about a minute to boot up. An onscreen message states that “power-on self-test passed.” Push “menu off” to clear the screen. The instrument is now ready to use.

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The Value of Hand-Held Oscilloscopes

The hand-held oscilloscope is a valuable asset for the advanced electrician or electronics technician. Its cost is a little more modest than a bench model with similar bandwidth, channel count, and features. However, most of the exercises in this article (including math and even the built-in spectrum analyzer) can be performed using a hand-held unit.

Moreover, the hazards discussed in this article in connection with touching an oscilloscope probe ground reference lead to a terminal that floats at a potential other than the premises electrical system ground level are not an issue for the hand-held, battery-operated oscilloscope. That is because all channel ground reference leads are isolated from system ground and from each other.

1. Measuring DC Voltage

Connect the probe tip and the ground reference lead to a suitable dry cell or 9V battery. Depending on the battery terminals, you may be able to use a hook-type probe tip and the ground reference lead’s alligator clip to complete the circuit. Alternately, a battery holder with stranded wire leads works well.

A slightly more elaborate version of this same exercise is to connect all four channels to separate 9V batteries. This would allow you to see separate traces for each power source. Each signal is color coded, which makes it easy to keep track of the signals. As a matter of fact, you could simply connect all four probes to a single DC battery, and the results would be the same.

Note: An oscilloscope is essentially an auto-ranging voltmeter, unless a specialized current probe is used. Therefore, in this exercise, you can expect to see a horizontal X-axis representing the passage of time, intersecting at the origin with a vertical Y-axis corresponding to amplitude ordinarily expressed as voltage with a specified number of volts represented by each division. The DC signal is displayed as a flat horizontal line above or below the X-axis, depending on the polarity.

2. Measuring Utility Voltage

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Another good exercise for getting started on the oscilloscope is to display the premises utility voltage. This is a worthwhile task because you can check the local power quality, which is often the first step in diagnosing sensitive electronic equipment problems.

If a variable-frequency motor drive is performing in an erratic fashion (e.g., motor overheating, stalling, or tripping out), a good way to start your troubleshooting endeavors is to look at the voltage, frequency, and waveform integrity of the power supply. This exercise simulates taking readings at the VFD input terminals, where a good sine wave of proper amplitude is needed. In performing this type of oscilloscope reading, there are some issues you must take note of, such as electric shock, arc flash, and improper connection of the ground reference lead to an input that floats above the ground potential of the oscilloscope’s AC supply.

In regard to electric shock and arc flash, it is necessary to provide live electrodes that can be accessed by the oscilloscope probe but cannot contact one another or be touched by the user/visitor to the work area. To this end, connect a polarized (!) plug to a length of flexible cord, terminated in a deep non-metallic wall box.

The black and white conductors should be different lengths (so they cannot possibly contact one another) and short enough so that they do not protrude much beyond the rim of the box. The wire ends should be stripped only long enough so that the bare stranded copper can be grabbed by the oscilloscope probe hook tip and ground reference lead alligator clip. The green equipment-grounding conductor should be cut back out of the way because it is not used in this demonstration. The polarity of this hookup should be checked with a voltmeter to make sure that the neutral (white) conductor does not float above ground potential. If white and black have become improperly crossed anywhere between the entrance panel and the plug and flexible cord, there will be a short circuit when the ground reference lead is connected — even before the probe is touched to the hot wire.

When the probe is connected to an analog channel input, the sine wave will display. You may have to press “autoset” to get a coherent reading.

As an additional exercise, plug an electric drill or other portable tool having a universal motor into a nearby receptacle on the same circuit, and squeeze the trigger to see if noise and/or distortion are introduced into the signal.

3. Measuring Utility Frequency

With the utility supply still connected to an analog channel, you can take advantage of the digital voltmeter (DVM) that is built into the oscilloscope. With the sine wave displayed, press “measure.” The measure menu appears across the bottom of the display. Press the soft key (so called because it corresponds to various items depending upon context) associated with DVM. The digital voltmeter menu appears to the right of the display. Use the top soft key, now labeled “mode,” in conjunction with Multipurpose Knob A to move the DVM to AC RMS. Notice at the top of the display this value is shown with slight real-time variations.

Then, use Multipurpose Knob A to access “frequency,” now displayed at the top. Real-time deviations are in the neighborhood of .01 Hz.

Note: Older analog CRT-type scopes had plastic overlays (called reticules) with horizontal and vertical lines parallel to the X- and Y-axes. The purpose was to allow technicians to count divisions and ticks so as to ascertain amplitude, peak-to-peak, and elapsed time for each cycle. Today’s flat-screen displays incorporate an electronic reticule instead. The digital voltage and frequency readouts provide greatly enhanced precision.

4. Saving a Waveform

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You can easily save any oscilloscope display — either to the internal memory of the unit or to an external medium such as a flash drive that is plugged into the front-panel USB port. To illustrate, turn on the Arbitrary Function Generator with the BNC cable in place, and activate the ramp waveform.

Note: It’s a good policy to default the instrument first and also press “autoset” to optimize the display.

Insert a flash drive into the USB port. Then, press “save.” The ramp waveform is now in the flash drive. Remove the flash drive, and take it to a remote computer. The flash drive icon will show up on the computer screen. Click on it, and a window will open showing the contents of the flash drive, including the ramp wave as displayed in the oscilloscope. The file can be renamed (if desired) and opened.

5. Remotely Accessing the Waveform

Today’s oscilloscopes can be connected to the Internet. Using the web browser in your computer, you can not only see the oscilloscope screen in close to real time, but also actually operate the controls. Here’s a simple way to do it.

Using the Ethernet port (marked LAN) on the back panel, run a Cat. 5e or later cable to your computer’s IP modem or Ethernet router, switch, or hub. The oscilloscope screen will display the DHCP status, including the instrument IP address — typically something like 192.168.1.67.

In your computer (with a web browser open), type the IP number into the address bar, and hit the return/enter key. The oscilloscope web-enabled user home page will appear. Click on “instrument control” to bring up the oscilloscope control page. Now the current oscilloscope screen is displayed on your computer screen.

Additionally, the oscilloscope can be remotely controlled by clicking on individual controls below the display. For example, with a BNC cable connecting the AFG output to analog Channel One in the oscilloscope, click on AFG in the subsystem section. Notice that the sine wave appears on the computer screen.

Now (on the computer screen) you can click on menu items such as “waveform.” Then, clicking on the arrow associated with a virtual Multipurpose Knob A, you can select any of the built-in waveforms. Waveform settings can also be selected and adjusted.

6. Adding Two Separate Signals

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Both bench-type and hand-held oscilloscopes incorporate interesting math functionality. To see how it works, display separate signals in two channels. Each trace will be color-coded to indicate the channel through which it is being accessed. Then, press the button labeled “math.” As always, this button will light up.

The math menu bar appears along the bottom of the display. Press the soft key that corresponds to Dual Waveform Math. First Source, associated with Multipurpose Knob A, and Second Source, associated with Multipurpose Knob B, are self-explanatory. Use the second soft key to scroll through the operators. This will give you a feel for how the four different operations of one signal upon the other will play out.

Note that in all cases the math result is shown in red, which is not one of the assigned channel colors.

7. AFG Functionality

As a very useful option, the oscilloscope incorporates an internal arbitrary function generator (AFG). Press “default setup.” With all probes disconnected, use a short coaxial cable equipped with BNC ends to connect the AFG output on the back panel to an active analog channel input. Then, press “AFG” to bring up the relevant menu.

Generally, Channel One and Sine Wave are default settings. Press the Waveform soft key at the bottom of the display. Multipurpose Knob A scrolls through the available waveforms. There are 13 permanent items in the internal library, plus Arbitrary, which is user generated.

Press the second soft key along the bottom, labeled “waveform settings.” A menu to the right of the display allows the user to adjust frequency or amplitude and other parameters using Multipurpose Knobs A and B or, much faster, the keypad.

Output settings (the bottom right soft key) permits additional refinements. For example, load impedance can be toggled, and Multipurpose Knob A will add various amounts of noise to the signal — a valuable troubleshooting tool.

8. Analysis in the Frequency Domain

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In the previous exercises, all waveforms were viewed in the time domain. What this means is that the vertical Y-axis represents amplitude expressed in volts, and the horizontal X-axis represents time (typically expressed in milliseconds). When the oscilloscope is switched over to the frequency domain mode, the Y-axis still represents amplitude but now reflects power in a logarithmic decibel scale instead of volts. The radical change, however, is in the horizontal X-axis, which now depicts frequency. In this mode, you can easily detect the relative strength of all harmonics at a glance.

The relationship between time and frequency domains is in accordance with some very complex mathematical relationships inherent in the Fourier Transform, and somewhat simpler Fast Fourier Transform (FFT), which can be rendered back and forth in the display by manipulating some simple controls. It is important to note that in moving back and forth between these two domains, information is neither gained nor lost. These are merely different views of the same data.

For a hands-on demonstration, first access AFG (viewed through an analog channel input), and make sure the sine wave is selected. Then, with the BNC cable still connected to the AFG output on the back panel, swing the other end over to the RF port at the right, using an RF adapter to make the connection. Then, just under the keypad, press “RF” to bring up that menu.

With sine wave accessed through the AFG, it is displayed in the frequency domain, as indicated by the single large spike at the left. The trace is easier to see if you press “freq/span” just under the RF button, then press the bottom soft key at the right associated with R to Center. Notice that all the power in the sine wave is concentrated in the fundamental with no harmonics. That is the nature of a sine wave.

Now, returning to AFG, switch to square wave. The frequency domain displays many strong harmonics outside of the fundamental. This is due to the square wave’s very fast rise and fall times.

9. Analyzing in Spectogram Mode

As part of the frequency display, a spectrogram can be viewed. Just as the FFT is a subdivision of the math functionality, the spectrogram is a subdivision of the frequency domain. It is still another permutation of values assigned for representation along the two axes. But now time is represented on the Y-axis and frequency on the X axis. But what about amplitude?

Three axes cannot readily be shown on a two-dimensional screen, so amplitude is represented by color. Hot colors correspond to a higher amplitude, and cooler colors represent a lower amplitude. Cool blue blends into the background, whose slightly grainy appearance is due to the noise floor of the oscilloscope.

With AFG displaying square wave, push RF once more to bring up the spectrum menu across the bottom of the screen. Press the second soft key from the left, labeled spectrogram, which is currently OFF. The spectrogram menu appears at the right. Toggle the spectrogram display ON. Now there is a split screen with frequency domain at the bottom and spectrogram at the top. Notice that it rises slowly from the bottom. That is because time is displayed along the Y-axis. Frequency is displayed along the X-axis, so there is a one-to-one correspondence between the two views.

The spectrogram is valuable for examining slowly changing RF phenomena. You can stop acquisitions and navigate through the spectrogram’s history by pressing “slice” in the side menu, then turning Multipurpose Knob A. You may want to switch AFG waveforms to check out their various harmonic components as shown in the spectrogram view.

10. Using the Triggering Function

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For the final exercise, let’s take a look at the triggering menu. With the BNC cable connected to the AFG port on the back panel, connect the other end to an analog channel input. Press “AFG,” and set the waveform to sine. Then, in the triggering section adjacent to the keypad, press the “trigger menu” button. Notice that the trigger type is “edge.” It is instructive to adjust the trigger level knob and watch the results.

Note: The default level is 0.00V. If you turn the knob slowly clockwise, you can watch the voltage increase, as shown by the changing numerical amount and also by the position of the horizontal trigger level line, which disappears after 2 seconds of inactivity.

The waveform shifts to the left, indicating the change along the time line of the triggering event. What is really interesting is the fact that when the horizontal line representing the triggering level becomes higher than the peak voltage, the trace abruptly becomes unstable — as if there were no triggering, which is, in fact, the case. Triggering is restored when the level is decreased below the waveform peak.

Triggering behavior can be controlled by other trigger menu options, and it is suggested that the user go through these categories. Adjust the parameters for the various AFG waveforms, and watch the results to see what conclusions can be drawn.

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