ETS-Lindgren HI-3637 VLF Magnetic Field Meter - Manual (Page 42)

Page — 34 HI n 3637 Manual
to capture all the important harmonics.
In addition to the horizontal sweep-frequency circuit,
there is a vertical deflection circuit which is used to
deflect the electron beam down the CRT screen and in so
doing produce characters. The vertical sweep frequency
is approximately 60 Hz.
The horizontal deflection circuit operates on the principle
that the force exerted on a moving electron is at right
angles to both the direction of the electron's motion and
the applied magnetic field. To induce a horizontal
component to the electron's original direction, the
magnetic field must possess a vertical polarization. Thus,
the horizontal deflection coils in VDT's and television
receivers tend to generate magnetic fields which are
strongly vertically polarized near the front of the screen.
RF fields caused by the deflection circuitry can produce
electric fields at normal operator positions of typically a
few V/m up to some tens of V/m and magnetic fields in
the range of a few mA/m up to several hundred mA/m
(Harvey, 1983b; Guy, 1987; Boivin, 1986; Joyner, et al,
1984; Marha and Charron, 1983). The HI-3637 is
designed specifically for measurement of the RF fields
associated with the beam deflection systems in VDT's
and television receivers.
Broadband RF Fields
An electronic clock within the VDT which typically
operates in the frequency range of 1 to 20 MHz is the
source of most of the radiated RF signals from the digital
electronics sub-section (Roy, et al, 1983). Conventional
shielding techniques are the usual method for eliminating
or reducing such emissions. Petersen, et al, (1980) and
Weiss and Petersen (1979) evaluated RF emissions from
a number of VDT's and found that RF electric field
strengths, measured at a distance of 1.5 meters from the
front of the VDT, for those emissions not associated with
the flyback circuit were well below 1 V/m RMS, typically
less than 0.01 V/m.