A&D Series 57ZZ Manual do Utilizador descarregar pdf (Página 9)

samples. It stores these samples until it accumulates

enough samples to describe a waveform, and then re-

assembles the waveform for viewing on the screen.

The conventional digitizing scope is known as a

DSO – Digital Storage Oscilloscope. Its display

doesn’t rely on luminous phosphor; instead, it uses a

raster-type screen.

Recently a third major oscilloscope architecture has

emerged: the Digital Phosphor Oscilloscope (DPO).

The DPO is a digitizing scope that faithfully

emulates the best display attributes of the analog

scope and provides the benefits of digital acquisition

and processing as well. Like the DSO, the DPO uses a

raster screen. But instead of a phosphor, it employs

special parallel processing circuitry that delivers a

crisp, intensity-graded trace.

For both DSOs and DPOs, the digital approach

means that the scope can display any frequency

within its range with equal stability, brightness, and

clarity. The digitizing oscilloscope’s frequency range

is determined by its sample rate, assuming that its

probes and vertical sections are adequate for the

task.

For many applications either an analog or digitizing

oscilloscope will do. However, each type has unique

characteristics that may make it more or less suitable

for specific tasks.

People often prefer analog oscilloscopes when it’s

important to display rapidly varying signals in “real

time” (as they occur). The analog scope’s chemical

phosphor-based display has a characteristic known

as “intensity grading” which makes the trace brighter

wherever the signal features occur most often. This

makes it easy to distinguish signal details just by

looking at the trace’s intensity levels.

Digital storage oscilloscopes allow you to capture

and view events that may happen only once – “tran-

sient” events. Because the waveform information is

in digital form (a series of stored binary values), it

can be analyzed, archived, printed, and otherwise

processed within the scope itself or by an external

computer. The waveform doesn’t need to be contin-

uous; even when the signal disappears, it can be

displayed. However, DSOs have no real-time inten-

sity grading; therefore they cannot express varying

levels of intensity in the live signal.

The Digital Phosphor Oscilloscope breaks down the

barrier between analog and digitizing scope tech-

nologies. It’s equally suitable for viewing high

frequencies or low, repetitive waveforms, transients,

and signal variations in real time. Among digitizing

scopes, only the DPO provides the Z (intensity) axis

that’s missing from conventional DSOs.

How Oscilloscopes Work

To better understand the oscilloscope’s many uses,

you need to know a little more about how oscillo-

scopes display a signal. Although analog oscillo-

scopes work somewhat differently than digitizing

oscilloscopes, some of the internal systems are

similar. Analog oscilloscopes are simpler in concept

and are described first, followed by a description of

digitizing oscilloscopes.

Analog Oscilloscopes

When you connect an oscilloscope probe to a circuit,

the voltage signal travels through the probe to the

vertical system of the oscilloscope. Figure 6 is a

simple block diagram that shows how an analog

oscilloscope displays a measured signal.

Depending on how you set the vertical scale

(volts/div control), an attenuator reduces the signal

voltage or an amplifier increases the signal voltage.

Next, the signal travels directly to the vertical deflec-

tion plates of the cathode ray tube (CRT). Voltage

applied to these deflection plates causes a glowing

dot to move. (An electron beam hitting the phosphor

inside the CRT creates the glowing dot.) A positive

voltage causes the dot to move up while a negative

voltage causes the dot to move down.

The signal also travels to the trigger system to start or

trigger a “horizontal sweep.” Horizontal sweep is a

term referring to the action of the horizontal system

causing the glowing dot to move across the screen.

Triggering the horizontal system causes the hori-

zontal time base to move the glowing dot across the

screen from left to right within a specific time

interval. Many sweeps in rapid sequence cause the

movement of the glowing dot to blend into a solid

line. At higher speeds, the dot may sweep across the

screen up to 500,000 times each second.

Together, the horizontal sweeping action and the

vertical deflection action traces a graph of the signal

Figure 5. Analog and digitizing oscilloscopes display waveforms.

Figure 6. Analog oscilloscope block diagram.

Delay Line

Trigger

Amp

Display

Vert

Amp

Horiz

Amp

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