Session 11

The purpose of this session is to teach you how to use an oscilloscope to measure steady voltage levels and electrical signals which are often also called waveforms.

Oscilloscope for web

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

  • Describe the difference between an electrical signal and a voltage level
  • Describe the function, features and characteristics of an oscilloscope
  • Use an oscillscope to measure the voltage growth across a capacitor using cursors to measure the change in voltage and the change in time

BTEC Outcomes Covered

  • Unit 53 Pass 1
  • Unit 6 Pass 5

What is an electrical signal or waveform

Perhaps we should start by defining what a signal actually is. The easiest way to do this is to use the lights on a car as an example.

When it is dark the headlights of the car will be switched on. A steady dc voltage will cause electrons to flow through the lamp element and produce incandescent light. The function of the light is to illuminate the road so that the driver can steer the car safely. However no information is conveyed

Car indicator lights, on the other hand, do produce a signal. The driver switches on the indicator and the light flashes on and off. It tells other road users that the car is going to change direction. In this example, information is conveyed.

To make the indicator light switch on and off we must have a voltage that is switching on and off. This is the essence of an electrical signal, it is simply a change in voltage over time. Here are some very common examples of electrical signals. Note that in each example:

  • Voltage is measured vertically
  • Time is measured horizontally

Electrical Signals

 Electrical signals can be very simple such as the examples above. Or enormously complex such as the signals transmitted and used by digital systems. A digital system uses a binary code, which is simply a voltage switched on and off in a sequence. The sequence used denotes the information that is being transmitted.

Digital bits

At this stage of our studies we are going to be dealing with simple electronic signals. Oscilloscopes allow us to see what is happening within a circuit at a particular point. You cannot go very far in practical electronics without learning how to use an oscilloscope.

Functions of an oscilloscope

You will be required to list the functions of an oscilloscope. Lets start with the most obvious one.

“An oscillscope is an electrical measurement instrument that draws a graph of voltage with respect to time and displays the result on a screen”

This allows us to:

  • Measure the amplitude (the size) of an electrical signal in volts
  • Measure the frequency of an electrical signal in Hertz (Hz)
  • Measure the mark/space ratio of an electrical signal
  • Measure the periodic time of an electrical signal
  • Measure and calculate the RMS value of an electrical signal
  • Compare 2 different electrical signals at different points in a circuit
  • Measure the difference in phase of an electrical signal between the inputs and output of a circuit or system

Features of an Oscilloscope

To allow the oscilloscope to carry out its functions it must have certain features 😉

Display technology allows us to see the actual graph of voltage over time. Modern scopes generally use LCD screens as they are far cheaper to make and mass produce. Older scopes made use of Cathode Ray Tubes (CRT) to display the electrical signal.

Input and output connectors the signal is transmitted to the the oscilloscope via special test probes. The probes are usually connected using a BNC connector. Most oscilloscopes have 2 channel inputs and an external sweep input. The scope will often have outputs, usually USB nowadays, so that the signal can be viewed on a computer or other device.

Attenuators, some input signals are too big for the oscilloscope to measure safely. Therefore they need to be attenuated (made smaller) before the scope can be connected. Attenuation is usually carried out by the oscilloscope probe. Signals can be attenuated by up to 1000 times (made 1000 times smaller). It is more usual however to attenuate a signal by 10x when attenuation is required.

AutoRanging or Autoset, sometimes we might have no information about the signal we have connected our scope up to measure. In this instance, pressing the Autoset button tells the oscilloscope to search for and display the connected signal. You will almost certainly need to tweak the voltage and time controls after using the Autoset button but it is a very useful tool in some circumstances.

Cursors allow the accurate measurement of amplitude and time elapsed at any point of a waveform.

Trigger Controls have two main purposes. The first is to ensure that the oscillscope displays a stable picture of the waveform by triggering the timebase for each sweep at the correct point. The other main use of the trigger control is to be able to capture and display sporadic fast transient signals which would otherwise be missed.

In built calibration facilities, whilst oscillscopes do need to be calibrated professionally, most modern scopes have a self calibration facility. Most scopes also offer the facility to calibrate the probe you are using to measure the signal.

Portability lots of modern scopes are very light or just connect directly to a laptop. This makes them very portable and flexible in comparison to the older CRT types.

Characteristics of an Oscilloscope

Input impedance, impedance is just another name for resistance. It is however to be precise, the electrical resistance of a circuit which is transimitting an ac waveform. Any equipment that is used to measure voltage must have a high input impedance. A typical input impedance for an oscilloscope is 1 Million Ohms. If higher input impedances are required then the probe attenuators can be adjusted.

Resolution is defined as being, the smallest change in signal which can be read on the oscilloscope screen. Resolutions of 1 or 2 millvolts (thousandths of a volt) are common for most scopes in general use. Resolution in terms of time ranges down to around 1 to 10 nanoSeconds for scopes in general usage.

Bandwidth, we say that the limit of an electrical signalling system is reached when the original signal has fallen to half of its original power or the -3dB point. Bandwidth is an important characteristic for an oscilloscope. It is a measure of how fast a signal the scope can see. A scope with a bandwidth of 40 Mhz (40 million cycles per second) can read a sine wave signal up to 40 Mhz. One should be aware of two things though.

  • The bandwidth for a square wave signal will be considerably less than the quoted bandwidth
  • The bandwidth of the scope is tied to the bandwidth of the probe. If you use poorly specced probes then don’t expect your scope to perform anywhere near to its maximum specification

We have briefly introduced some of the major functions, features and characteristics of the most important piece of electronic measurement equipment. However the best way to learn about an oscilloscope it to actually use one.

Using an Oscilloscope

You are now going to use an oscilloscope to measure the waveform (electrical signal) across a charging capacitor. You will already be familiar with the circuit as we used it in the last session.

Here is a video of the process

Procedure

  • Set up a power supply to give 5 volts dc. Make sure the current is fully switched on, dial fully clockwise. Then switch off the power supply.
  • Connect a 100,000 ohm resistor and a 100uF capacitor in series to the power supply

Capacitor charge circuit

Set up the oscilloscope so that it has:

  • A timebase of 10 seconds
  • An amplitude of 1 volt/division
  • Coupling set to dc

Connect the probe across the capacitor:

probe connect

A physical example

Physical example

  • Switch on the power supply and use the cursors to measure the voltage after 1 time constant has elapsed (10 seconds)
  • Calculate the percentage of the maximum voltage measured after 1 time constant
  • Compare your measured results with your calculated results for 5 time constants

This concludes all of the work required for the session.

 

 

 

 

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