ETS-Lindgren HI-3637 VLF Magnetic Field Meter - Manual (Page 39)

HI n 3637 Manual Page — 31
APPENDIX A
VIDEO DISPLAY TERMINALS (VDTs)
VDT CHARACTERISTICS
General Description
Video display terminals (VDT's) and television receivers
are quite similar in certain respects. Both are used to
display information; the VDT displaying information
received from a computer system, word processing
system, or other digital information system and the
television receiver displaying video information
transmitted from television broadcast stations. In
conjunction with a keyboard, the VDT serves as the main
interface between the operator and a word processor,
computer, etc. Television receivers are sometimes used
in lieu of VDT's with home computer systems.
Principles of Operation
VDT's and television receivers use the same basic
principles of operation. Both contain a large evacuated
glass tube called a cathode-ray tube (CRT), or picture
tube in the case of television receivers. The CRT
contains a source of electrons (the cathode) at one end
and a fluorescent coating on the inside of the viewing
screen. Electrons released from the cathode are
accelerated by a high voltage (typically in the range of 10
to 25 kilovolts) and are projected onto the fluorescent
material of the screen which then emits visible light when
it is struck by the fast-moving electrons. The CRT also
includes various electrodes for focusing the electron
beam and for scanning the beam across the fluorescent
screen. Electronic circuitry in the VDT modulates the
electron beam to produce the intended images on the
screen. This circuitry leads to the production of
electromagnetic fields (emissions). There are four basic
aspects to the electrical environment of VDT emissions:
(1) 60/50 Hz modulated DC fields; (2) 60/50 Hz fields;
(3) RF fields associated with the horizontal and vertical
deflection systems; (4) broadband RF fields caused by the
digital electronic circuits which are associated with
character generation (Roy, et al, 1983).