ETS-Lindgren HI-3603 VLF Survey Meter - Manual (Page 24)

HI-3603 VDT/VLF Survey Meter
© ETS-Lindgren, August, 2005
Revision E, Part # H-600042
24
When the displayed value is below 5% of the current full scale,
the arrow at the left end of the BarGraph display will indicate.
When the "Below Range" arrow is indicating, the accuracy of the
reading may not be within the specified tolerance. When
possible, the scale should be changed to permit a normal field
strength reading without the "Below Range" indication. If the
displayed value is too high, the " Over Range" arrow at the right
end of the BarGraph, will indicate. Select the next appropriate
scale.
VDT CHARACTERISTICS
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.
PRINCIPLE 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