The Oscilloscope

The above vid-clip shows how to use an oscilloscope in detail

(but remember - he is American and America has a 60Hz mains AC supply - whereas we have a 50Hz one in the UK!)

An oscilloscope is basically a voltmeter that shows you how voltage varies with time... it plots a voltage against time graph on the screen.

It is connected in parallel to the component you are looking at (like a voltmeter).

 

Instead of getting a digital readout (as on a multimeter) it gives you a graph.

  • The y-axis is voltage (so you can see how many volts are across the component).
  • The x-axis is time (so you can see whether the voltage is steady (D.C.) or varying (A.C.))

This is most useful when you look at AC voltages.

 

You can switch the x-axis on or off using the timebase control dial – and change the scale of the ‘graph’ too using this dial.

You can change the y-axis scale using the voltage gain dial.

When you change the settings the graph looks different but you haven’t changed the supply voltage – just what the graph of it looks like.

You should be able to work out the frequency from the period by using

f = 1/T

Here is a link to a site that will tell you how to set one up and how it works ... and below are some brief notes from me...

 

You should know the basics:

graphic from doctronics

The above graphic is adapted from doctronics.co.uk (see above link)

  • A heated electrode gives off electrons (thermionic emission).

  • If these are accelerated across a vacuum (must be a vacuum otherwise they would just ionize the air!)

  • by a potential difference (they would be pulled towards a positive plate).

  • They can be directed at a fluorescent screen and where they hit it will light up - photons of visible light emitted

  • If the electron beam has the voltage you are investigating put across it (on the Y plates) it will be pulled towards the +ve one (bigger the voltage the bigger the pull!... so the further up the screen the beam will move)

  • Across the screen a sawtooth wave pulls the spot from left to right steadily (at a speed shown on the timebase dial) and then flips it back to the left again to start again.
If the timebase is off you just get a spot - you can vary its size using the focus and intensity controls - you shouldn't leav it on like this for a long time as it will 'burn out the screen'  - affect the zinc suphide coating
If the timebase is on at a good speed you get a line because the fluorescence doesn't have time to die away before the screen is hit again!

If a DC voltage is applied across the Y-plates when the timebase is off then the steady voltage makes the spot be a fixed distance higher than its rest position and you get a spot (above or below) the no signal spot.

You can measure the voltage by working out how much it has 'jumped up' and converting the divisions on the screen to volts.

It is good practice to make it jump up - measure the voltage and then switch the contacts round - making it jump down - and measure the voltage again - you should get the same result!

If a DC voltage is applied across the Y-plates when the timebase is on then the steady voltage makes the line be a fixed distance higher than its rest position and you get a horizontal line (above or below) the no signal line.

You measure the voltage in the same way as you would using the 'spot'

If an AC voltage is applied across the Y-plates when the timebase is off then the sinusoidally varying voltage makes the spot move up and down around its rest position and you get a vertical line through and centring on the no signal spot. (From this you can work out the peak to peak voltage).

Remember that the peak to peak voltage has to be halved to give you the peak voltage - and that has to be divided by root 2 to give you the RMS voltage!

If an AC voltage is applied across the Y-plates when the timebase is on then the sinusoidally varying voltage makes the spot move up and down around its rest position as it moves across the screen and you see a sine wave graph. (From this you can work out the period and hence the frequency of the signal - do it across several periods on different timebase settings to double check your readings).

Click here for a page of questions to try

Additional AS Level Oscilloscope questions to try

1. Why should you maximize the volts/division scale when determining the amplitude of a wave form?

2. What effect does changing the volts/division scale from 1 volt per division to 5 volts per division have on the display of the incoming wave on the screen? (Does the wave amplitude increase, decrease or remain the same?)


3. What effect does changing the volts/division scale from 1 volt per division to 5 volts per division have on the incoming wave itself?


4. One cycle of a waveform occupies five divisions of an oscilloscope screen. The timebase dial is set to 1 ms/div. Calculate the frequency.


5. One cycle of a waveform occupies two divisions of an oscilloscope screen. The timebase dial is set to 5 ms/div. Calculate the frequency.


6. A waveform occupies six divisions of an oscilloscope screen when the timebase is switched off. The voltage gain is set to 0.5V/div. Calculate the peak to peak current being tested if the resistance of the circuit is known to be 10 ohm.


7. A waveform occupies two divisions of an oscilloscope screen when the timebase is switched off. The voltage gain is set to 10 mV/div. Calculate the peak current being tested if the resistance of the circuit is known to be 1 k W.


8. A waveform occupies six divisions of an oscilloscope screen when the timebase is switched off. The voltage gain is set to 0.25V/div. Calculate the rms current being tested if the resistance of the circuit is known to be 19 ohm.


9. A waveform occupies three divisions of an oscilloscope screen when the timebase is switched off. The voltage gain is set to 20 mV/div. Calculate the rms current being tested if the resistance of the circuit is known to be 1.4 kW .


10. Sketch an oscilloscope screen showing a 200 Hz frequency, 4V rms trace. Include all calculations you have done to work out what you should draw, and don't forget to indicate clearly what the timebase and voltage gain settings are!

LOJ January 2002- revised 2008