ELECTROMAGNETIC DEFLECTION

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 focus. Deflection Systems 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 and electrostatic deflection. 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 deflection. 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 CRT’S.– 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 scanning system. Raster Scanning 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 vertical retrace. 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 1-3