THE CRO - an introduction


A Cathode Ray Oscilloscope
Photograph © Rapid Electronics

The symbol for a CRO:

THE CRO SCREEN
The screen of a CRO is very similar to a TV, except it is much simpler. We will not go into the similarities except to say that the "picture tube" on a TV and the "screen" on a CRO are both a special type of valve called a Cathode Ray Tube.
It is a vacuum tube with a cathode (negative electrode) at one end that emits electrons and  anodes (positive electrodes) to accelerate the electron beam up/down and left/right to hit a phosphor coating at the end of the tube, called the screen. This is shown in the animation:

The electrons are called cathode rays because they are emitted by the cathode and this gives the oscilloscope its full name: Cathode Ray Oscilloscope or CRO.
The CRO screen is overlaid (covered) with a grid of squares (lines) - generally at 1cm spacing. These lines are called GRATICULES - pronunced: 'gratu`kyools
The CRO tube produces an electron beam that is fired at the screen to produce a dot of illumination (usually  green or blue) at the centre of the screen.
This dot it then moved across the centre of the screen by a circuit called the Vertical Sweep. The speed of the dot is determined by a multi-position switch labelled: "Time/Div." If the switch is set to "1s," the dot will move 1cm (one graticule-marking) across the screen in one second.

This is very slow and as the switch is clicked to 0.5s, 0.2s, 0.1s, the speed of the dot increases and the result is a green line or "trace" across the centre of the screen. If the switch is clicked to other values such as 1mS, 0.1mS, 10uS, the dot will move too fast to be observed and a straight line will result, across the centre of the screen.


When a signal is applied to a CRO via the "probe," the voltage will be passed though the Vertical Amplifier and the dot will be raised or lowered and the combination of the CRO moving the dot across the screen and the input voltage moving it up and down, produces a "picture" called the waveform or "trace."
If the input voltage is "AC" the shape will be a sinewave as shown in the graph below:

There are hundreds of wave-shapes and the following are just two:

 

The screen can be considered to be a piece of graph paper, with an X-axis and Y-axis. You can see the centre-lines are thicker. This is just to help counting the squares and to quickly position the trace in the centre of the screen.
The X-axis is the horizontal axis and in our case it is the "time" axis.
The Y-axis (the up-down axis) is the voltage axis. If the trace is moving across the centre of the screen and the input voltage produces a waveform above the centre-line, the voltage is said to be "positive." When the waveform is below the centre-line, the voltage is "negative."

If the voltage is only positive, and the waveform rises above the top of the screen, it can be moved down by turning the "POSITION" control.

This means the 0v or X-axis will be the lowest marking across the screen. That's why the screen is not marked with any values. The 0v can be the lowest line across the screen or the highest, depending on the polarity of the voltage you are measuring.

If the voltage is not "rising and falling" but steady, it is called "DC" and the CRO will show the waveform as a steady line (trace) at a distance above the centre-line (if the voltage is positive), or below the line if the voltage is negative. In these measurements, the CRO is like a voltmeter.

A dual trace oscilloscope can display two traces on the screen at the same time, allowing comparison of two different signals, such as the input and output of an amplifier. Always consider buying and using a dual trace CRO. 


A Dual trace CRO

  • An oscilloscope must be handled carefully to protect its fragile tube.
  • Oscilloscopes use high voltages on the tube and these remain for some time after switching off . Take care when working on the circuitry inside a CRO.

These are just of the waveforms you will observe on a CRO. There is a lot more to getting a steady image on the screen.
One of the biggest secrets to getting a steady image is "triggering."
These skills are a long way off yet.
The next thing we will do is use an interactive program to show some of the waveforms and how they appear when the time control  and volts/div is adjusted.


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