Specifications

8
What Is an Oscilloscope and Why Do You Need One? (continued)
Types of oscilloscopes
Types of oscilloscopes
Analog oscilloscopes
The first oscilloscopes were
analog oscilloscopes, which use
cathode-ray tubes to display a
waveform. An electron beam is
scanned across a series of many
horizontal lines while being gated
on and off. Photoluminescent
phosphor on the screen
illuminates when an electron
hits it, and as successive bits of
phosphor light up, you can see
a representation of the signal.
A trigger is needed to make
the displayed waveform look
stable. When one whole trace
of the display is completed, the
oscilloscope waits until a specific
event occurs (for example, a
rising edge that crosses a certain
voltage) and then starts the trace
again. An untriggered display is
unusable because the waveform is
not shown as a stable waveform
on the display (this is true for
DSO and MSO oscilloscopes,
which will be discussed below,
as well).
Analog oscilloscopes are useful
because the illuminated phosphor
does not disappear immediately.
You can see several traces of
the oscilloscope overlapping
each other, which allows you to
see glitches or irregularities in
the signal. Since the display of
the waveform occurs when an
electron strikes the screen, the
intensity of the displayed signal
correlates to the intensity of the
actual signal. This makes the
display act as a three-dimensional
plot (in other words, x-axis is
time, y-axis is voltage, and z-axis
is intensity).
The downside of an analog
oscilloscope is that it cannot
“freeze” the display and keep the
waveform for an extended period
of time. Once the phosphorus
substance deluminates, that part
of the signal is lost. Also, you
cannot perform measurements
on the waveform automatically.
Instead you have to make
measurements by hand using
the grid on the display. Analog
oscilloscopes are also very
limited in the types of signals
they can display because there
is an upper limit to how fast the
horizontal and vertical sweeping
of the electron beam can occur.
While analog oscilloscopes are
still used by many people today,
they are not sold very often.
Instead, digital oscilloscopes are
the modern tool of choice.
Digital storage oscilloscopes (DSOs)
Digital storage oscilloscopes
(often referred to as DSOs) were
invented to remedy many of
the negative aspects of analog
oscilloscopes. DSOs input a
signal and then digitize it through
the use of an analog-to-digital
converter. Figure 12 shows an
example of one DSO architecture
used by Agilent digital
oscilloscopes.
The attenuator scales the
waveform. The vertical amplifier
provides additional scaling while
passing the waveform to the
analog-to-digital converter (ADC).
The ADC samples and digitizes
the incoming signal. It then stores
this data in memory. The trigger
looks for trigger events while
the time base adjusts the time
display for the oscilloscope. The
microprocessor system performs
any additional postprocessing
you have specified before the
signal is finally displayed on the
oscilloscope.
Having the data in digital
form enables the oscilloscope
to perform a variety of
measurements on the waveform.
Signals can also be stored
indefinitely in memory. The data
can be printed or transferred to a
computer via a flash drive, LAN,
or DVD-RW. In fact, software
now allows you to control and
monitor your oscilloscope from
a computer using a virtual front
panel.
Channel
Input
Attenuater
Vertical
amplifier
ADC MegaZoom
Micro-
processor
Display
Channel
memory
Trigger Time-base
Figure 12. Digitizing oscilloscope architecture
Oscilloscope Fundamentals