Application Note

ManualsBrandsFluke ManualsPortable OscilloscopesFluke 123

A rst look at DSOs

This introduction to digital storage oscilloscopes

(DSOs) takes you on a quick but comprehensive

tour of DSO functions and measurements.

An oscilloscope measures and

displays voltage signals on a

time-versus-voltage graph. In

most applications the graph

shows how the signal changes

over time: the vertical (Y) axis

represents voltage, and the hori-

zontal (X) axis represents time.

This simple graph can tell you

many things about a signal:

• View the signal for anomalies.

• Calculate the frequency of an

oscillating signal.

• Tell if a malfunctioning compo-

nent is distorting the signal.

• Tell how much of the signal is

noise and whether the noise is

changing with time.

Today’s handheld digital stor-

age oscilloscopes offer two

critical advantages over benchtop

models: they are battery-oper-

ated, and they use isolated,

electrically floating inputs.

These designs make safety-

certified measurements possible

in 1000 V CAT III and 600 V

CAT IV environments—a critical

need for safely troubleshooting

electrical devices in high-energy

applications.

Scopes and DMMs

The difference between an

oscilloscope and a DMM (Digital

Multimeter) can be summarily

stated as “pictures vs. numbers.”

A DMM is a tool for making

precise measurements of discrete

signals, enabling readings of up

to eight digits of resolution for

the voltage, current or frequency

of a signal. On the other hand, it

cannot depict waveforms visually

to reveal signal strength, wave-

shape, or the instantaneous value

of the signal. Nor is it equipped to

reveal a transient or a harmonic

signal that could compromise the

operation of a system.

A scope adds a wealth of infor-

mation to the numeric readings of

a DMM. While displaying instan-

taneous numerical values of a

wave, it also reveals the shape of

the wave, including its ampli-

tude (voltage) and frequency.

With such visual information, a

transient signal that may pose

major consequences to a system

can be displayed, measured and

isolated.

Reach for a scope if you want

to make both quantitative and

qualitative measurements. Use

a DMM to make high-precision

checks of voltage, current,

resistance and other electrical

parameters.

Sampling

Sampling is the process of

converting a portion of an input

signal into a number of discrete

electrical values for the pur-

pose 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.

The input waveform appears as

a series of dots on the display. If

the dots are widely spaced and

difficult to interpret as a wave-

form, they can be connected

using a process called interpola-

tion, which connects the dots

with lines, or vectors.

Volts

vertical

Y-axis

Time

horizontal X-axis

Figure 1. Time-versus-voltage graph.

The 4-channel Fluke 190 Series II

ScopeMeter has a 200 MHz bandwidth

and 2.5 GS/s real-time sampling rate.

The Fluke 123 ScopeMeter with 20 MHz

dual-input measurement shows both

meter reading and waveform.

Application Note

From the Fluke Digital Library @ www.fluke.com/library

Summary of content (6 pages)

PAGE 1
A first look at DSOs This introduction to digital storage oscilloscopes (DSOs) takes you on a quick but comprehensive tour of DSO functions and measurements. Application Note An oscilloscope measures and displays voltage signals on a time-versus-voltage graph. In most applications the graph shows how the signal changes over time: the vertical (Y) axis represents voltage, and the horizontal (X) axis represents time. This simple graph can tell you many things about a signal: • View the signal for anomalies.
PAGE 2
Triggering Trigger controls allow you 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.
PAGE 3
However, simply by manually adjusting the trigger point to a repetitive, unique point on the edge, you can solve this problem and produce a stable waveform display. Now adjust the vertical sensitivity, expanding the waveform vertically but ensuring that the high and low points of the waveform do not exceed the vertical span. Trigger position. Sets the trigger position to the 50 % point of the vertical amplitude.
PAGE 4
Understanding and reading waveforms The majority of electronic waveforms encountered are periodic and repetitive, and they conform to a known shape. Here are the factors to consider in analyzing waveforms: 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. Amplitude. Verify that the level is within the operating specifications of the circuit.
PAGE 5
Ringing. Ringing can be seen mostly in digital circuits and in radar and pulse-width-modulation applications. Ringing shows up at the transition from a rising or falling edge to a flat dc level. Check for excessive ringing, adjusting the time base to give a clear depiction of the transitioning wave or pulse. 110 Vac Figure 16b. Using an oscilloscope with a paperless recorder mode allows plotting of the amplitude (voltage level) over time. Figure 15.
PAGE 6
Troubleshooting Whichever troubleshooting scenario below is appropriate at the time, remember that it’s important to inspect waveforms for fast-moving transients or glitches, even if a spot check of the waveform reveals no anomalies. These events can be difficult to spot, but the high sampling rate of today’s DSOs, together with effective triggering, makes it possible. DUT with KGU. This approach assumes that you have access to a KGU and a reference library. 1.