voltage of the control grid varies to control the
strength of the beam. The stronger the beam, the
brighter the display is on the phosphor screen.
The screen grid voltage remains constant and acts
as an accelerator for the beam. A negative charge on
the focus grid shapes the electrons into a beam.
Varying the charge of the focus grid causes the
diameter of the beam to vary to determine optimum
The deflection system in a CRT moves the beams
to create the display.
Two common types of
deflection systems are used in CRTs. These are
ELECTROMAGNETIC DEFLECTION. Elec-
tromagnetic deflection uses a magnetic field generated
by four coils to move the beam across the CRT.
Electromagnetic deflection is commonly found on
CRTs that use a raster-scan type display.
Current flows through the electron beam as it
moves from the electron gun (cathode) to the
phosphor face (anode) of the CRT. This current
develops a circular magnetic field. By introducing an
external magnetic field, the beam can be deflected.
Controlling the polarity and strength of this external
field controls the amount and direction of the beam
The magnetic field is introduced into the CRT by
the yoke assembly. The yoke consists of four coils of
wire mounted at 90-degree increments. The yoke is
mounted around the neck of the CRT. Current
flowing through the coil produces a magnetic field at
a right angle to the coil. The magnetic field will cause
the electron beam to deflect.
ELECTROSTATIC DEFLECTION CRTS.
Electrostatic-type deflection CRTs are generally used
in radar and oscilloscopes.
In the electrostatic
deflection CRT, four deflection plates are located
inside the CRT. The top and bottom plates control
vertical deflection of the beam and the right and left
plates control the horizontal deflection of the beam.
An electrical charge is applied to these plates to direct
the beam to the proper area of the CRT. To move the
beam to the right, a positive charge is applied to the
right plate to pull the beam while a negative charge is
applied to the left plate to push the electron beam to
the proper position.
The amount of the charge
applied to the plates controls the amount of deflection.
CRT SCANNING METHODS
The creation of a display is known as a scan. Two
types of scanning systems are currently in use in
CRTs: raster scanning and vector scanning. Raster
scan CRTs are commonly used with electromagnetic
deflection CRTs. Vector scan CRTs are commonly
used with electrostatic deflection systems, although
either deflection system can be used with either
A raster scan CRT develops the display or picture
by painting a series of horizontal lines across the face
of the CRT. The electron beam is pulled from left to
right. The beam is then turned off and the horizontal
deflection voltage returns the beam to the left side,
and the vertical deflection voltage pulls the beam
down one line space.
The left to right motion is the horizontal frequency
and is much greater than the top to bottom motion or
vertical frequency. The time it takes for the beam to
return to the left or top of the screen is known as
retrace time. During retrace the beam is blanked.
By dividing the horizontal frequency by the
vertical frequency, we can determine the maximum
number of lines in the raster. Standard television uses
15,750 Hz for the horizontal frequency and 60 Hz for
the vertical frequency. Using this formula, we find
that the maximum number of lines is 262.5; but some
lines are not available because of the time required for
The lines are spaced close enough to each other so
the eye cannot detect any variation of intensity.
Resolution is the number of lines per inch at the