A&D Series 57ZZ Manual do Utilizador descarregar pdf (Página 11)

“cell” of information for every single pixel in the

scope’s display. Each time a waveform is captured

(in other words, every time the scope triggers), it is

mapped into the Digital Phosphor database’s cells.

Each cell representing a screen location that is

touched by the waveform gets reinforced with inten-

sity information. Others do not. Thus intensity infor-

mation builds up in cells where the waveform passes

most often.

When the Digital Phosphor database is fed to the

oscilloscope’s display, the display reveals intensified

waveform areas, in proportion to the signal’s

frequency of occurrence at each point – much like

the intensity grading characteristics of an analog

oscilloscope (unlike an analog scope, though, the

DPO allows the varying levels to be expressed in

contrasting colors if you wish). With a DPO, it’s easy

to see the difference between a waveform that occurs

on almost every trigger and one that occurs, say,

every 100th trigger.

Importantly, the DPO uses a parallel processing

architecture to achieve all this manipulation without

slowing down the whole acquisition process. Like

the DSO, the DPO uses a microprocessor for display

management, measurement automation, and

analysis. But the DPO’s microprocessor is outside the

acquisition/display signal path (see Figure 9), where

it doesn’t affect the acquisition speed.

Sampling Methods

Digitizing oscilloscopes – DSO or DPO – can use

either real-time, interpolated real-time, or equiva-

lent-time sampling to collect sample points. Real-

time sampling is ideal for signals whose frequency is

less than half the scope’s maximum sample rate.

Here, the oscilloscope can acquire more than enough

points in one “sweep” of the waveform to construct

an accurate picture (see Figure 10). Note that real-

time sampling is the only way to capture single-shot

transient signals with a digitizing scope.

When measuring high-frequency signals, the oscillo-

scope may not be able to collect enough samples in

one sweep. There are two solutions for accurately

acquiring signals whose frequency exceeds half the

oscilloscope’s sample rate:

• Collect a few sample points of the signal in a

single pass (in real-time mode) and use interpola-

tion to fill in the gaps. Interpolation is a

processing technique to estimate what the wave-

form looks like based on a few points

• Build a picture of the waveform by acquiring

samples from successive cycles of the waveform,

assuming the signal repeats itself (equivalent-time

sampling mode)

Real-Time Sampling with Interpolation

Digitizing oscilloscopes take discrete samples of the

signal which can be displayed. However, it can be

difficult to visualize the signal represented as dots,

especially because there may be only a few dots

representing high-frequency portions of the signal.

To aid in the visualization of signals, digitizing oscil-

loscopes typically have interpolation display modes.

In simple terms, interpolation “connects the dots.”

Using this process, a signal that is sampled only a

few times in each cycle can be accurately displayed.

However, for accurate representation of the signal,

the sample rate should be at least four times the

bandwidth of the signal.

Linear interpolation connects sample points with

straight lines. This approach is limited to recon-

Figure 9. Digital phosphor oscilloscope block diagram – “Parallel Processing.”

Acquisition

Rasterizer

Amp

Display

A/D

µP

Display

Memory

Digital

Phosphor

DPX

Figure 10. Real-time sampling.

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