PARTS OF OSCILLOSCOPE DEFINITION AND BASICS TUTORIAL

The most important part of an oscilloscope is a large, cone-shaped glass tube with a vacuum inside. It operates the way the old "vacuum tube" radios worked, before the invention of transistors. That is, there is a tungsten wire "filament" at the small end, which is heated by a low voltage but high current source, which could be a battery.

Electrons tend to fly off this hot wire, almost like vapor from a boiling liquid. The battery and filament assembly is the left-hand part of Fig. 8.

Since the electrons are negatively charged, they get strongly attracted to a wire hoop which has a high positive charge on it, from another power source, shown here as thousand-volt battery. (This would work, but in a regular oscilloscope, 120 volt ac from a wall socket is changed into thousand-volt dc by methods that will be explained later in the course.)

Some of the electrons get collected by the "anode" hoop and returned to the battery, but many are going so fast that their momentum tends to keep them going to the right, rather than changing direction quickly enough to get caught by the hoop, so they fly through the large hole in the middle. Since the beam of electrons is coming from the negatively charged filament (the "cathode" of the 1,000V circuit), this beam is called the "cathode ray." The whole tube is sometimes called a "cathode ray tube," or "CRT."

It can be an important part of a TV receiver, in which case it is called the "picture tube." The electrons continue until they eventually hit the "screen," which has a thin coating of "phosphor" material, such as zinc silicate. This contains impurities that convert the kinetic energy of the electrons into a visible spot of light, and that can be seen outside, through the glass tube.

The inside of the tube also has a thin conductive coating on it, to carry the electrons back to the power supply. A grid of thin wires is attached to a "terminal" outside the tube (the small white circle), and if a strong negative voltage is applied to the grid, it can stop or at least decrease the intensity of the visible light spot.

This is not always used in an oscilloscope, but it is an important part of a TV picture tube. Four metal plates are on the top, bottom, and sides of the tube, and they are all connected to outside terminals (only two of which are shown in this diagram, for simplicity).

If the top one is slightly charged + and the bottom one –, via the "vertical" terminals, the electron beam (cathode ray) is attracted upward, as shown in the diagram, and the spot of light thus appears up high on the screen. Similarly, if the "horizontal" plates are charged by some outside voltage, the spot will move to the side (not shown here).