Make Sense of Electrical Signals

Waveforms translate to crucial information about the health of your electrical/mechanical equipment systems. How do you read them?

Edited by Jane Alexander, Managing Editor

Devices that convert electrical power to mechanical power run the industrial world. Think pumps, compressors, motors, conveyors, and robots. Voltage signals that control these electro-mechanical devices are a critical but unseen force. The question is, how do you capture and  and anlyze that unseen force?

Oscilloscopes (or scopes) test and display voltage signals as waveforms, i.e., visual representations of the variation of voltage over time. The signals are plotted on a graph, which shows how the signal changes. The vertical (Y) access represents the voltage measurement and the horizontal (X) axis represents time.

According to the technical experts at Fluke Corp., Everett, WA, an oscilloscope graph can reveal important information, including:

• voltage and current signals when equipment is operating as intended
• signal anomalies
• calculated frequency of an oscillating signal and any variations in frequency
• whether a signal includes noise and changes to the noise.

Most of today’s oscilloscopes are digital—which enables more detailed, accurate signal measurements and fast calculations, data-storage capabilities, and automated analysis. Handheld digital oscilloscopes offer several advantages over benchtop models. They are battery operated, use electrically isolated floating inputs, and offer the advantage of embedded features that make oscilloscope usage relatively easy and accessible to a variety of workers.

Oscilloscope functions

Sampling. This is the process of converting a portion of an input signal into a number of discrete electrical values for the purpose of storage, processing, and display. The magnitude of each sampled point is equal to the amplitude of the input signal at the time the signal is sampled.

Fig. 1. Sampling and interpolation: Sampling is depicted by the dots while interpolation
is shown as the black line.

The input waveform appears as a series of dots on the display (Fig. 1). If the dots are widely spaced and difficult to interpret as a waveform, they can be connected using a process called interpolation, which connects the dots with lines, or vectors.

Fig. 2. Unknown trace adjusted for 3 to 6 vertical divisions.

Fig. 3. Unknown trace adjusted for 3 to 4 periods horizontally.

Fig. 4. Trigger point is set to the 50% point but, due to the aberration on the leading edge in the second period, an additional trigger results in an unstable display.

Fig. 5. Trigger level adjusted to a unique repetitive position, outside the aberration on the second period.

Triggering. Trigger controls allow users to stabilize and display a repetitive waveform.

Edge triggering is the most common form of triggering. In this mode, the trigger level and slope controls provide the basic trigger-point definition. The slope control determines whether the trigger point is on the rising or the falling edge of a signal, and the level control determines where on the edge the trigger point occurs.

When working with complex signals such as a series of pulses, pulse-width triggering may be required. With this technique, the trigger-level setting and the next falling edge of the signal must occur within a specified time span. Once these two conditions are met, the oscilloscope triggers.

Single-shot triggering is a technique by which the oscilloscope displays a trace only when the input signal meets the set trigger conditions. Once the trigger conditions are met, the oscilloscope acquires and updates the display, and then freezes the display to hold the trace.

Fig. 6. If the two waveform components aren’t symmetrical, there may be a problem with the signal.

Fig. 7. Use cursors and the gridlines to evaluate the rise and fall times of the leading and trailing edges of a waveform.

Fig. 8. Use horizontal cursors to identify amplitude fluctuations.

Getting a signal on the screen. The task of capturing and analyzing an unknown waveform on an oscilloscope can be routine, or it can seem like taking a shot in the dark. In many cases, taking a methodical approach to setting up the oscilloscope will capture a stable waveform or help you determine how the scope controls need to be set so that you can capture the waveform.

The traditional method of getting a signal to show properly on an oscilloscope is to manually adjust three key parameters to try to achieve an optimum set-point—often without knowing the correct variables:

• vertical sensitivity: Adjust the vertical sensitivity so that the vertical amplitude spans approximately three to six divisions.
• horizontal timing: Adjust the horizontal time per division so that there are three to four periods of the waveform across the width of the display.
• trigger position: Set the trigger position to the 50% point of the vertical amplitude. Depending on the signal characteristics, this action may or may not result in a stable display.

These three parameters, when adjusted properly, show you a symmetrical “trace,” the line that connects the samples of the signal to create the visual depiction of the waveform. Waveforms can vary indefinitely from the most common sine wave that ideally mirrors between positive and negative on the zero axis point or a unidirectional square wave typical of electronic pulses, or even a shark-tooth form.

The manual setup method often requires tediously adjusting the settings to make the waveform readable in order to analyze it. In contrast, some modern oscilloscopes automate the process of digitizing the analog waveform to see a clear picture of the signal.

Fig. 9. Evaluate waveform DC offsets.

Fig. 10. Evaluate period-to-period time changes.

Fig. 11. A transient is occurring on the rising edge of a pulse.

Understanding and reading waveforms

The majority of electronic waveforms encountered in the workplace are periodic and repetitive—and they conform to a known shape. As you train your eye to understand these waveforms, consider their varying dimensions:

• Shape. Repetitive waveforms should be symmetrical. That is, if you were to print the traces and cut them in two like-sized pieces, the two sides should be identical. A point of difference could indicate a problem.
• Rising and falling edges. Particularly with square waves and pulses, the rising or falling edges of the waveform can greatly affect the timing in digital circuits. It may be necessary to decrease the time per division to see the edge with greater resolution.
• Amplitude. Verify that the level is within the circuit operating specifications. Also check for consistency, from one period to the next. Monitor the waveform for an extended period of time, watching for any changes in amplitude.
• Amplitude offsets. DC-couple the input and determine where the ground reference marker is. Evaluate any DC offset and observe if this offset remains stable or fluctuates.
• Periodic wave shape. Oscillators and other circuits will produce waveforms with constant repeating periods. Evaluate each period in time using cursors to spot inconsistencies.

Fig. 12. This figure shows ground reference-point measurement indicating induced random noise.

Fig. 13. Excessive ringing occurring on the top of the square wave.

Fig. 14. This pattern shows a momentary change of approximately 1.5 cycles in the amplitude of the sine wave.

Fig. 15. Performing a frequency measurement on a crystal oscillator that has been trend-plotted over an extended period can highlight the effect of drift caused by temperature changes and aging.

Waveform anomalies

The following items reflect typical anomalies that may appear on a waveform, along with the typical sources of such anomalies.

• Transients or glitches. When waveforms are derived from active devices such as transistors or switches, transients or other anomalies can result from timing errors, propagation delays, bad contacts, or other phenomena.
• Noise. Noise can be caused by faulty power-supply circuits, circuit overdrive, crosstalk, or interference from adjacent cables. Or, noise can be induced externally from sources such as DC-DC converters, lighting systems, and high-energy electrical circuits.
• Ringing. Ringing is seen mostly in digital circuits and in radar and pulse-width-modulation applications. It shows up at the transition from a rising or falling edge to a flat DC level. To check for excessive ringing, adjust the time base to give a clear depiction of the transitioning wave or pulse.
• Momentary fluctuation. Momentary changes in the measured signal generally result from an external influence such as a sag or surge in the main voltage, activation of a high-powered device that is connected to the same electrical circuit, or a loose connection.
• Drift. Manifested as minor changes in a signal’s voltage over time, drift can be tedious to diagnose. Often the change is so slow that it is difficult to detect. Temperature changes and aging can affect passive electronic components such as resistors, capacitors, and crystal oscillators. One problematic fault to diagnose is drift in a reference DC voltage supply or oscillator circuit. Often, the only solution is to monitor the measured value (VDC, Hz). MT

This article was edited using information supplied by technical experts at Fluke Corp.,  Everett, WA. For more information on this and other testing and measurement topics and technologies, visit fluke.com.

Diagnosing Problems and Troubleshooting

Technical experts at Fluke Corp., note that while successful troubleshooting is an art and a science, adopting a methodology and relying on the functionality of an advanced oscilloscope can greatly simplify the process.

The time-tested approach known as KGU (known good unit) comparison builds on a simple principle: An electronic system that is working properly exhibits predictable waveforms at critical nodes within its circuitry, and these waveforms can be captured and stored.

A reference library of waveforms of a KGU can be stored on some oscilloscopes or printed out to serve as a hard-copy reference document. If the system or an identical system later exhibits a fault or failure, waveforms can be captured from the faulty system—called the device under test (DUT)—and compared with their counterparts in the KGU. Consequently, the DUT can either be repaired or replaced.

CAUTION: For the correct and safe use of electrical test tools, it is essential for operators to follow safety procedures as outlined by their company and local safety agencies.