Learning a new skill often involves learning a new vocabulary. This idea
holds true for learning how to use an oscilloscope. This section describes
some useful measurement and oscilloscope performance terms.
The generic term for a pattern that repeats over time is a wave - sound
waves, brain waves, ocean waves, and voltage waves are all repeating
patterns. An oscilloscope measures voltage waves. One cycle of a wave is
the portion of the wave that repeats. A waveform is a graphic representation
of a wave. A voltage waveform shows time on the horizontal axis and voltage
on the vertical axis.
Waveform shapes tell you a great deal about a signal. Any time you see a
change in the height of the waveform, you know the voltage has changed.
Any time there is a flat horizontal line, you know that there is no change for
that length of time. Straight diagonal lines mean a linear change - rise or
fall of voltage at a steady rate. Sharp angles on a waveform mean sudden
change. Figure 1 shows common waveforms and Figure 2 shows some
common sources of waveforms.
Figure 1: Common Waveforms
Figure 2: Sources of Common Waveforms
Types of Waves
You can classify most waves into these types:
- Sine waves
- Square and rectangular waves
- Triangle and sawtooth waves
- Step and pulse shapes
The sine wave is the fundamental wave shape for several reasons. It has
harmonious mathematical properties - it is the same sine shape you may
have studied in high school trigonometry class. The voltage in your wall
outlet varies as a sine wave. Test signals produced by the oscillator circuit of
a signal generator are often sine waves. Most AC power sources produce
sine waves. (AC stands for alternating current, although the voltage
alternates too. DC stands for direct current, which means a steady current
and voltage, such as a battery produces.)
The damped sine wave is a special case you may see in a circuit that
oscillates but winds down over time.
Figure 3 shows examples of sine and damped sine waves.
Figure 3: Sine and Damped Sine Waves
Square and Rectangular Waves
The square wave is another common wave shape. Basically, a square wave
is a voltage that turns on and off (or goes high and low) at regular intervals.
It is a standard wave for testing amplifiers - good amplifiers increase the
amplitude of a square wave with minimum distortion. Television, radio, and
computer circuitry often use square waves for timing signals.
The rectangular wave is like the square wave except that the high and low
time intervals are not of equal length. It is particularly important when
analyzing digital circuitry.
Figure 4 shows examples of square and rectangular waves.
Figure 4: Square and Rectangular Waves
Sawtooth and Triangle Waves
Sawtooth and Triangle waves result from circuits designed to control
voltages linearly, such as the horizontal sweep of an analog oscilloscope or
the raster scan of a television. The transitions between voltage levels of
these waves change at a constant rate. These transitions are called ramps.
Figure 5 shows examples of sawtooth and triangle waves.
Figure 5: Sawtooth and Triangle Waves
Step and Pulse Shapes
Signals such as steps and pulses that only occur once are called
single-shot or transient signals. The step indicates a sudden change in
voltage, like what you would see if you turned on a power switch. The pulse
indicates what you would see if you turned a power switch on and then off
again. It might represent one bit of information traveling through a computer
circuit or it might be a glitch (a defect) in a circuit.
A collection of pulses travelling together creates a pulse train. Digital
components in a computer communicate with each other using pulses.
Pulses are also common in x-ray and communications equipment.
Figure 6 shows examples of step and pulse shapes and a pulse train.
Figure 6: Step, Pulse, and Pulse Train Shapes
Waveform Measurements
You use many terms to describe the types of measurements that you take
with your oscilloscope. This section describes some of the most common
measurements and terms.
Frequency and Period
If a signal repeats, it has a frequency. The frequency is measured in Hertz
(Hz) and equals the number of times the signal repeats itself in one second
(the cycles per second). A repeating signal also has a period - this is the
amount of time it takes the signal to complete one cycle. Period and
frequency are reciprocals of each other, so that 1/period equals the
frequency and 1/frequency equals the period. So, for example, the sine
wave in Figure 7 has a frequency of 3 Hz and a period of 1/3 second.
Figure 7: Frequency and Period
Voltage is the amount of electric potential (a kind of signal strength) between
two points in a circuit. Usually one of these points is ground (zero volts) but
not always - you may want to measure the voltage from the maximum peak
to the minimum peak of a waveform, referred to at the peak-to-peak
voltage. The word amplitude commonly refers to the maximum voltage of a
signal measured from ground or zero volts. The waveform shown in Figure 8
has an amplitude of one volt and a peak-to-peak voltage of two volts.
Phase
Phase is best explained by looking at a sine wave. Sine waves are based on
circular motion and a circle has 360 degrees. One cycle of a sine wave has
360 degrees, as shown in Figure 8. Using degrees, you can refer to the
phase angle of a sine wave when you want to describe how much of the
period has elapsed.
Figure 8: Sine Wave Degrees
Phase shift describes the difference in timing between two otherwise similar
signals. In Figure 9, the waveform labeled "current" is said to be 905 out of
phase with the waveform labeled "voltage," since the waves reach similar
points in their cycles exactly 1/4 of a cycle apart (360 degrees/4 = 90
degrees). Phase shifts are common in electronics.
Figure 9: Phase Shift
The terms described in this section may come up in your discussions about
oscilloscope performance. Understanding these terms will help you evaluate
and compare your oscilloscope with other models.
The bandwidth specification tells you the frequency range the oscilloscope
accurately measures.
As signal frequency increases, the capability of the oscilloscope to
accurately respond decreases. By convention, the bandwidth tells you the
frequency at which the displayed signal reduces to 70.7% of the applied
sine wave signal. (This 70.7% point is referred to as the "-3 dB point," a
term based on a logarithmic scale.)
Rise Time
Rise time is another way of describing the useful frequency range of an
oscilloscope. Rise time may be a more appropriate performance
consideration when you expect to measure pulses and steps. An
oscilloscope cannot accurately display pulses with rise times faster than the
specified rise time of the oscilloscope.
The vertical sensitivity indicates how much the vertical amplifier can amplify
a weak signal. Vertical sensitivity is usually given in millivolts (mV) per
division. The smallest voltage a general purpose oscilloscope can detect is
typically about 2 mV per vertical screen division.
For analog oscilloscopes, this specification indicates how fast the trace can
sweep across the screen, allowing you to see fine details. The fastest sweep
speed of an oscilloscope is usually given in nanoseconds/div.
Gain Accuracy
The gain accuracy indicates how accurately the vertical system attenuates
or amplifies a signal. This is usually listed as a percentage error.
Time Base or Horizontal Accuracy
The time base or horizontal accuracy indicates how accurately the horizontal
system displays the timing of a signal. This is usually listed as a percentage
error.
Sample Rate
On digital oscilloscopes, the sampling rate indicates how many samples per
second the ADC (and therefore the oscilloscope) can acquire. Maximum
sample rates are usually given in megasamples per second (MS/s). The
faster the oscilloscope can sample, the more accurately it can represent fine
details in a fast signal. The minimum sample rate may also be important if
you need to look at slowly changing signals over long periods of time.
Typically, the sample rate changes with changes made to the sec/div control
to maintain a constant number of waveform points in the waveform record.
ADC Resolution (Or Vertical Resolution)
The resolution, in bits, of the ADC (and therefore the digital oscilloscope)
indicates how precisely it can turn input voltages into digital values.
Calculation techniques can improve the effective resolution.
Record Length
The record length of a digital oscilloscope indicates how many waveform
points the oscilloscope is able to acquire for one waveform record. Some
digital oscilloscopes let you adjust the record length. The maximum record
length depends on the amount of memory in your oscilloscope. Since the
oscilloscope can only store a finite number of waveform points, there is a
trade-off between record detail and record length. You can acquire either a
detailed picture of a signal for a short period of time (the oscilloscope "fills
up" on waveform points quickly) or a less detailed picture for a longer period
of time. Some oscilloscopes let you add more memory to increase the
record length for special applications.
Next Chapter: Setting Up
Previous Chapter: The Oscilloscope
Table of Contents: Table of Contents