THE CATHODE-RAY OSCILLOSCOPE

Electrostatic Deflection and Focus Control

The cathode ray tube consists essentially of an "electron gun" for producing a beam of rapidly moving electrons called cathode rays,

A fluorescent screen upon which a luminous spot is produced by the impact of the cathode rays,

and a means for displacing the spot from its quiescent position as the result of current or voltage applied to the deflecting mechanism.

Deflecting Electrons

As you should know, there are two ways to move an electron (and thus an electron beam): either with a magnetic or with an electrostatic field.

Because of this, there are three possible ways to move or deflect an electron beam in a CRT:

Magnetically, Electrostatically. Electromagnetically

much of the test equipment uses electrostatic deflection.

ELECTROSTATIC DEFLECTION uses principles you are already familiar with. Namely, opposites attract, and likes repel.

In oscilloscope CRTs, electrostatic deflection is used to deflect the beam is horizontally by applying an electric field between a pair of plates to its left and right, and vertically by applying an electric field to plates above and below.

How Does It Work?

A charged particle experiences a force when it is in a region where there exists an electric field. From the definition of the electric field, this force is given by Equation 1.

When an electron (q = -e) is in an electric field, the electron experiences a force in the direction opposite of the electric field.

At the cathode, the electrons are initially at rest. By applying a potential difference Vacc, between the cathode and Anode 2, an electric field, given by Equation 2, is established between the cathode and Anode2.

The kinetic energy of the electrons as they pass through Anode 2, after traveling a distance Δx is

hence

Once the electrons leave the electron gun at a speed of (vx ), the electric fields of the

deflecting plates will “steer” the electron beam.

The electron then travels between the deflection plate, which have a voltage of Vd across them as seen above. The electric field within the plates is given by:

Note: The electricField beyond thePlates is essentially Zero.

This electric field will exert a force on the electron, opposite the direction of the arrows in Figure 2, causing the trajectory of the electron to be altered, as seen in Figure 3.

F

How much does it deflect?

The magnitude of deflection

of the electron y is given by

The electron travels under

the plate for a period of tp .

As the initial y-velocity and displacement

equals to zero

The acceleration of theElectron is given by

recall

When the electron leaves the plates region, its y-velocity is

Which equals to

The time of travel between the plates and the screen is:

Hence

Therefore, the total deflection is given by:

Sensitivity We now define the deflection

sensitivity

It is desirable for a tube to be as sensitive as possible The deflection plates of the C.R.T. is connected to

amplifiers. To enable the amplifiers to have as high bandwidth as possible, the capacitance between the plates should be kept low (L small, d large). However, this would decrease the sensitivity.

We will sacrifice sensitivity for bandwidth. Practical CRTs are therefore the result of a compromise.

To Sum up

In the CRT the electric field is created by two sets of paired electrodes, mounted at right angles, that the electron stream flows between. This arrangement allows independent deflection of the beam in two dimensions (usually perceived as up/down (vertical) and right/left (horizontal))

The Focus Control

This control adjusts CRT focus to obtain the sharpest, most-detailed trace.

This is accomplished my means of focusing anodes (a part of the electron gun) which acts as an “electrostatic lens”.

The Electrostatic Lens

An electrostatic lens is a device that assists in the transport of charged particles, and used to focus an ion beam.

Specifically, we're interested in Einzel lens.

An einzel lens is a charged particle lens that focuses without changing the energy of the beam.

It consists of three or more sets of cylindrical or rectangular tubes in series along an axis. It is used in ion optics to focus ions in flight which is accomplished through manipulation of the electric field in the path of the ions.

The einzel lens principle is used as a focusing mechanism in modern display and television cathode ray tubes, and has the advantage of providing a good sharply focused spot throughout the useful life of the tube's electron gun.

This is the resulting phenomena

Focusing of a beam of electrons The motion of electrons in the cathode-ray tube of an oscilloscope

Thank You

By Issa Jawabreh