Using an Oscilloscope Electricity Lesson 12

Homework Revise for the exam! Make revision check lists (lists of definitions, equations, derivations), make sure you know these  easy marks in the exam. Practice questions, past papers, out of the book, go on the website if you need more.

Learning Objectives To know what an oscilloscope is and how it works. To know how to measure the pd of alternating current and direct current. To know how to measure the frequency of an alternating current.

Cathode Ray Oscilloscope

Definition From the specification book:- An oscilloscope consists of a specially made electron tube and associated control circuits. The electron gun emits electrons towards a fluorescent screen  light is emitted when electrons hit the screen  this is what we see.

Tube Photograph

Electron Gun Photograph

Tube Diagram H.T. supply heater supply y plates +- phosphor screen anode x plates

How does it work? When no p.d. is applied across the plates the spot on the screen is stationary. If a pd is applied across the X-plates the beam of electrons is deflected horizontally and the spot moves across. pd across Y-plates  spot moves up and down.

Oscilloscope Controls The x-plates are connected to a time base circuit which is designed to make the spot move across the screen in a given time  then back again much faster.  a bit like a trace on a heart monitor. The y-plates are connected to the Y-input and this causes the spot to move up or down depending on the input pd.

summary H.T. supply heater supply y plates +- phosphor screen anode electron gun produces a beam of electrons light produced on the screen by electron beam a p.d. across the y plates deflects the trace vertically a p.d. across the x plates deflects the trace horizontally x plates

Oscilloscope Controls The gain sets the scale for the y-axis, normally in volts per cm. The time base sets the scale for the x-axis, normally in ms per cm. Recall that frequency can be calculated from the period from the graph using:

Gain and Time-Base Controls

Peak Voltage Peak p.d. = 3 Divisions x 1.0 mV/div = 3.0 mV

Period & Frequency period = 4.0 divisions x 1.0 ms/div = 4.0 ms frequency = 1 / period frequency = 1 / s frequency = 250 Hz

Self Test

Application Measuring the speed of ultrasound. Set up the oscilloscope so that the time base circuit triggers a transmitter to send out a pulse of ultrasonic waves. The receiver can be connected to the Y-input of the oscilloscope so that the waveform can be seen on the screen when it is detected.

Measuring Speed of Ultrasound

Calculating Speed The time difference between the transmission of the pulse (left hand side of screen) and it being received (peak of waveform) can be measured from the screen, using the timebase. If the distance between the transmitter and the receiver is known, then the speed can be calculated using:-