cathode ray oscilloscope
A cathode ray oscilloscope (often called a CRO) is an instrument which can be used to measure voltages and/or frequencies of electrical signals. It is widely used in scientific measurement and research.
 |
Oscilloscopes are have a display screen, and a panel of control knobs and input sockets. The signal to be measured is fed to one of the input connectors, using a coaxial cable, or a 'scope probe' |  |
Because so many physical quantities can be converted to a voltage, a CRO can be used to display and measure such diverse quantities as temperature, sound, velocity and light intensity. It does so by plotting a two-dimensional graph of the voltage of one or more electrical signals (vertical axis) plotted usually as a function of time (horizontal axis).
 |
In this representation of a trace on a CRO, the Potential (voltage) varies from -10V to +10V approx. many times per second. |
A grid is drawn on the screen of a CRO to help with measurement.
Each square of the grid is known as a division.
| Functions of an oscilloscope |
Trace
In its simplest mode, the oscilloscope repeatedly draws a horizontal line called the trace across the middle of the screen from left to right. The timebase control, sets the speed at which the line is drawn, and is calibrated in seconds per division.
| Shows the effect of changing the time base control on CRO when there is no input voltage. |
If the input voltage is not zero, the trace is deflected either upwards or downwards. The vertical control, sets the scale of the vertical deflection, and is calibrated in volts per division. The resulting trace is a graph of voltage against time obtained just by setting the timebase to match the frequency of the input signal.
Many osilloscopes have two or three channels so that signals can be input on different channels and compared.
 |
Half Wave Rectification produced by a diode, displayed on top of origianl alternating current (shown by dashed lines). |
if the input signal is a 20 Hertz sine wave, then its period is 50 ms, so the timebase should be adjusted so that the time between successive horizontal sweeps is 50 ms. Unfortunately, an oscilloscope's timebase is not perfectly accurate, and the frequency of the input signal is not perfectly stable, so the trace will drift across the screen making measurements difficult.
| Shows the effect of changing the time base control on the display when there is an input voltage varying in time. |
If the input voltage is of higher frequency, then the trace may well be a sine wave.
| Shows the effect of changing the time base control on the display when there is an input voltage varying in time when the frequency of the voltage is high. |
Voltage Control
This affects the amplitude of the signal displayed on the screen of the CRO.
| Shows the effect of changing the voltage control on the display. |
Trigger
To provide a more stable trace, modern oscilloscopes have a function called the trigger. When using triggering, the scope will pause each time the sweep reaches the extreme right side of the screen. The scope then waits for a specified event before drawing the next trace. The effect is to prevent horizontal drift of the trace. In this way, triggering allows the display of periodic signals such as sine waves.
| Showing the effect of changing the trigger level on the display. |
If you wish to review the functions of the CRO, try the demonstration below:
| Functions of an oscilloscope |
Television
