Oscilloscope

Muhajir Ab. RahimSchool of Mechatronic Engineering

Universiti Malaysia Perlis

Introduction

• The oscilloscope is basically a graph-displaying device.

• It draws a graph of an electrical signal.

• In most applications the graph shows how signals change over time.

• The vertical (Y) axis represents voltage and the horizontal (X) axis represents time

This simple graph can tell you many things about a signal. • You can determine the time and voltage values of a signal.• You can calculate the frequency of an oscillating signal.• You can see the "moving parts" of a circuit represented by the signal.• You can tell if a malfunctioning component is distorting the signal.• You can find out how much of a signal is direct current (DC) or

alternating current (AC)

Types of Oscilloscope

There are two types of oscilloscope1. Analog oscilloscope works by directly applying a voltage

being measured to an electron beam moving across the oscilloscope screen. The voltage deflects the beam up and down proportionally, tracing the waveform on the screen. This gives an immediate picture of the waveform.

2. Digital oscilloscope samples the waveform and uses an analog-to-digital converter (or ADC) to convert the voltage being measured into digital information. It then uses this digital information to reconstruct the waveform on the screen

Analog Digital

So what is the advantages?

• People often prefer analog oscilloscopes when it is important to display rapidly varying signals in "real time" (or as they occur).

• Digital oscilloscopes allow you to capture and view events that may happen only once. They can process the digital waveform data or send the data to a computer for processing. Also, they can store the digital waveform data for later viewing and printing.

How does an Oscilloscope work?

How does an Oscilloscope work?

• Like a televison screen, the screen of an oscilloscope consists of a cathode ray tube.

• Although the size and shape are different, the operating principle is the same. Inside the tube is a vacuum.

• The electron beam emitted by the heated cathode at the rear end of the tube is accelerated and focused by one or more anodes, and strikes the front of the tube, producing a bright spot on the phosphorescent screen.

• The electron beam is bent, or deflected, by voltages applied to two sets of plates fixed in the tube.

• The horizontal deflection plates, or X-plates produce side to side movement.

• As you can see, they are linked to a system block called the time base. This produces a sawtooth waveform.

• During the rising phase of the sawtooth, the spot is driven at a uniform rate from left to right across the front of the screen. During the falling phase, the electron beam returns rapidly from right ot left, but the spot is 'blanked out' so that nothing appears on the screen.

How does an Oscilloscope work?

• The signal to be displayed is connected to the input. • The AC/DC switch is usually kept in the DC position (switch closed)

so that there is a direct connection to the Y-amplifier• In the AC position (switch open) a capacitor is placed in the signal

path, in order to block DC signals but allows AC signals to pass. • The trigger circuit is used to delay the time base waveform so that

the same section of the input signal is displayed on the screen each time the spot moves across. The effect of this is to give a stable picture on the oscilloscope screen, making it easier to measure and interpret the signal.

What will signal look like?• The oscilloscope has an illuminated dot that moves across the

screen.  With no signal, it would look like the following.

What will signal look like?• When a sinusoidal signal is applied, then the vertical position is

proportional to the voltage at any instant.  If you applied a low frequency sine signal, you would get a track like the one below

What will signal look like?• If you have a sinusoidal signal that repeats every half millisecond - a

frequency of 2kHz - you would get a picture like this one.  • It would appear to be stationary on the oscilloscope screen, but it

really isn't.  It's just that it repeats so frequently that you see it as a constant image

How to use an Oscilloscope?

To use an oscilloscope, you need to adjust some basic settings to accommodate an incoming signal:

• The attenuation or amplification of the signal using the volts/div control to adjust the amplitude of the signal before it is applied.

• The time base using the time/div control to set the amount of time per division represented horizontally across the screen.

• The triggering of the oscilloscope using the trigger level to stabilize a repeating signal, as well as triggering on a single event.

• Adjusting the contrast enables you to create a sharp, visible display.

How to use an Oscilloscope?• The front panel is divided into

three main sections labelled Vertical, Horizontal, and Trigger. Your oscilloscope may have other sections, depending on the model and type.

• Notice the input connectors on your oscilloscope. This is where you attach probes. Most oscilloscopes have at least two input channels and each channel can display a waveform on the screen. Multiple channels are handy for comparing waveforms, usually called ch1 and ch2.

Triggering Demo

Measuring Time Demo

Measuring Voltage Demo

Further Reading

https://www.cs.tcd.ie/courses/baict/bac/jf/labs/scope/index.html