ETS-Lindgren HI-3637 VLF Magnetic Field Meter - Manual (Page 40)

Page — 32 HI n 3637 Manual
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 provides
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).