ETS-Lindgren HI-3603 VLF Survey Meter - Manual (Page 25)

HI-3603 VDT/VLF Survey Meter
© ETS-Lindgren, August, 2005
Revision E, Part #H-600042 25
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).
MODULATED DC FIELDS
To accelerate the electron beam toward the screen, a high DC
voltage is used. The high voltage is produced by pulsing a
transformer which has a high turns ratio and is often derived
from the deflection circuitry, though in some cases it may have a
higher frequency depending on the character display system.
The drive pulse is a square wave which produces a high voltage
secondary pulse that is rich in harmonic content. The AC
components of this DC current pulse flow to ground via the
capacitance formed by the CRT screen and the resistive coating
on the outside of the CRT. This small capacitance provokes the
filtering necessary for a smooth high voltage accelerating
potential. Roy et. al (1983) have reported that one method of
reducing the AC component of the DC field is to place an RC
filter network between the high voltage transformer output and
the CRT. They found that such a filter could, in some VDT's,
reduce the AC component of the DC field by as much as 50 dB
(a factor of over 300 times).
The modulated DC field is produced by the charge on the face of
the CRT and is largely confined to the front of the unit. This field
is highly variable, being affected by humidity, capacitance
between the CRT and external objects and touching the CRT
(Harvey, 1984a). Several investigators have measured the
strength of this DC field and found values ranging from a few
hundred volts per meter to as high as 45 kV/m at the surface of
the body of an operator, and depending on the proximity of the
operator to the VDT, closer distances resulting in higher
measured incident DC fields (Olsen, 1981;Harvey, 1984b;Nylen
et al., 1984;Bracken et al.,1985).