Different Types of Oscilloscopes Available in Market

Types of Oscilloscopes

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There are basically two types of oscilloscopes available in the market :

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  1. Analog Oscilloscope
  2. Digital Oscilloscope

Though for many applications, either an analog or digital oscilloscope will do. However, each type has unique characteristics that may make it more or less suitable for specific applications.

Digital oscilloscopes can be further classified into the following :

  • Digital storage oscilloscopes (DSOs)
  • Digital phosphor oscilloscopes (DPOs)
  • Sampling oscilloscopes

Analog Oscilloscopes

Fundamentally, an analog oscilloscope works by applying the measured signal voltage directly to the vertical axis of an electron beam that moves from left to right across the oscilloscope screen – usually a cathode-ray tube (CRT).

The back side of the screen is treated with luminous phosphor that glows wherever the electron beam hits it.

The signal voltage deflects the beam up and down proportionally as it moves horizontally across the display, tracing the waveform on the screen. The more frequently the beam hits a particular screen location, the more brightly it glows.

The CRT limits the range of frequencies that can be displayed by an analog oscilloscope.

At very low frequencies, the signal appears as a bright, slow-moving dot that is difficult to distinguish as a waveform.

At high frequencies, the CRT’s writing speed defines the limit. When the signal frequency exceeds the CRT’s writing speed, the display becomes too dim to see.

The fastest analog oscilloscopes can display frequencies up to about 1 GHz.

When you connect an oscilloscope probe to a circuit, the voltage signal travels through the probe to the vertical system of the oscilloscope. Figure 1 illustrates how an analog oscilloscope displays a measured signal.

The architecture of an analog oscilloscope

The architecture of an analog oscilloscope

Depending on how you set the vertical scale (volts/div control), an attenuator reduces the signal voltage and an amplifier increases the signal voltage.

Next, the signal travels directly to the vertical deflection plates of the CRT.

Voltage applied to these deflection plates causes a glowing dot to move across the screen.

The glowing dot is created by an electron beam that hits the luminous phosphor inside the CRT. 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 refers to the action of the horizontal system that causes the glowing dot to move across the screen. Triggering the horizontal system causes the horizontal 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 per second.

Together, the horizontal sweeping action and the vertical deflection action trace a graph of the signal on the screen. The trigger is necessary to stabilize a repeating signal – it ensures that the sweep begins at the same point of a repeating signal, resulting in a clear picture as shown in Figure 2.

The trigger stabilizes a repetitive waveform, creating a clear picture of the signal.

The trigger stabilizes a repetitive waveform, creating a
clear picture of the signal.

In addition, analog oscilloscopes have focus and intensity controls that can be adjusted to create a sharp, legible display.

People often prefer analog oscilloscopes when it is important to display rapidly varying signals in “real time” – or, as they occur.

The analog oscilloscope’s chemical phosphor-based display has a characteristic known as intensity grading that makes the trace brighter wherever the signal features occur most often. This intensity grading makes it easy to
distinguish signal details just by looking at the trace’s intensity levels.

Digital Oscilloscopes

In contrast to an analog oscilloscope, a digital oscilloscope uses an analog-to-digital converter (ADC) to convert the measured voltage into digital information.

It acquires the waveform as a series of samples, and stores these samples until it accumulates enough samples to describe a waveform.

The digital oscilloscope then re-assembles the waveform for display on the screen. (see Figure 3)

Analog oscilloscopes trace signals, while digital oscilloscopes sample signals and construct displays.

Analog oscilloscopes trace signals, while digital oscilloscopes
sample signals and construct displays.

Digital oscilloscopes can be classified into :

  1. Digital storage oscilloscopes (DSOs)
  2. Digital phosphor oscilloscopes (DPOs)
  3. Sampling oscilloscopes

The digital approach means that the oscilloscope can display any frequency within its range with stability, brightness, and clarity.

For repetitive signals, the bandwidth of the digital oscilloscope is a function of the analog bandwidth of the front-end components of the oscilloscope, commonly referred to as the –3dB point.

For single-shot and transient events, such as pulses and steps, the bandwidth can be limited by the oscilloscope’s sample rate.

We will discuss about each type of digital oscilloscope in our next article(Different types of digital Oscilloscopes).