Session 12

Overview

In this session we will be defining the term virtual instrument, learning about the parameters of a sine wave and the purpose and applications of a function generator. We will then use the function generator to test a simple low pass filter and measure the input and output of the filter circuit using an oscilloscope.

The oscilloscope will pass the signal to a computer using a USB cable and we will use the software controls to capture, measure and analyse the input and output to the circuit with reference to theory

By the end of this session the learner will be able to:

  • Describe the parameters of a sine wave
  • Describe the function, features and characteristics of a function generator
  • Test an rc circuit using a function generator and virtual oscilloscope in terms of amplitude and phase shift
  • Describe the measurement techniques and hardware, software requirments needed to use the laboratory oscilloscopes as virtual instruments

BTEC Outcomes Covered

  • Unit 53 Pass 2
  • Unit 53 Pass 7
  • Unit 53 Pass 8

What is a Virtual Instrument

In terms of computer think about the differences betweeh hardware and software. Hardware comprises of things such as the motherboard, hard drive and peripheral devices such as printers. Software is a set of instructions in the form of a program which the hardware carries out to perform a huge variety of tasks such as music production, film production, circuit simulation and virtually any other number of tasks you can think of.

Hardware is so fast and software so flexible that, in terms of electrical and electronic measurement, many of the measurements are increasingly being taken by virtual instruments.

A virtual measurement instrument is simply a software emulation of a physical measurement instrument. All that is really  required is a means of connecting the electrical signal to be measured to the computer, in other words an interface.

Most modern oscilloscopes have an interface which allows them to be used as a virtual instrument. The signal is fed to the computer via a USB cable, there are of course other ways of connecting the signal to a computer such as firewire but USB is thought to be the most common method. Here is the general arrangement including the circuit you are going to be measuring today:

Virtual instrument setup

The parameters of a sine wave

Before we can begin todays task we need to introduce some theory, namely the parameters of a sine wave. This video explains the most common and important parameters you will need to know about at this of your studies.

 

The following diagram explains the key concepts

Sinewave parameters diagram

Amplitude or Peak Value

The highest value the waveform reaches on each positive cycle (the value above the centre or neutral line).

Periodic time or Wavelength

The amount of time required for a sine wave to complete 1 full cycle (360 degrees)

Frequency

The amount of time a waveform completes a full cycle every second:

Frequency = 1/Periodic time

Frequency is measured in Hertz (Hz)

RMS Value

The heating effect produced by a sine wave when compared to the heating value produced by a dc supply. For a sine wave this is always given as:

RMS = 0.707 x Peak Value

Peak to Peak Value

The distance between the maximum positive and negative values of the sine wave

Phase

The following diagram explains the key points of phase and phase difference

Phase

For more information on sine waves and their properties you can visit this link

Practical Task

Measure the input and output of a single phase ac circuit using a virtual oscilloscope. The following schematic shows the connections required.

Single phase schematic

  • Set the function generator to give a sine wave output of 1 kHz
  • Set the amplitude of the signal to be 10 volts peak to peak
  • Connect the function generator to the oscilloscope
  • Set up the oscilloscope to read the signal
  • Set up the oscilloscope to measure the output signal
  • When the oscilloscope is successfully measuring the input and output signals connect it to a computer and startup the oscilloscope software
  • Compare the input signal to the output signal in terms of amplitude using the software cursors
  • Adjust the frequency of the function generator to the point that the output signal = 0.707 x the input signal
  • Measure the phase difference between the input and output signals using the software cursors

This video describes the process of setting up the oscilloscope and function generator

Once your oscilloscope and function generator are set up you can test the circuit and measure the input and output on the scope. This following video shows how.

Finally we need to transfer the signal to a computer so that the signal can be measured in a virtual environment shown next.

Written Tasks

Unit 53 Pass 2

Describe the function, features and characteristics of a function generator

By this stage of your studies you should know how to describe each of the above given the previous examples we have given with an oscilloscope and a bench power supply. Describe the function generator used for this sessions tests as your example

Unit 53 Pass 7

Use a virtual measurement and test system to carry out a test on a piece of electronic equipment

Describe the process used to test the low pass filter and take measurements from it using the oscilloscope software controls on the computer. Photographs, illustrations, screen dumps and calculations are all required to complete this section.

Unit 53 Pass 8

Describe the measurement techniques, instrument connection, hardware and software used

Measurement techniques is a description of what values were measured and how the data was gathered. For example how the oscilloscope and function generator were set up to conduct the test

Instrument connection deals with how the function generator was connected to your circuit, how the oscilloscope was connected to take measurements and how the oscilloscope was interfaced with the computer

You should give a brief description of the hardware and software used to carry out this test. Hardware to include the physical oscilloscope and computer. Software to include the OWON Virtual Oscilloscope

This concludes all of the work required for this session

 

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