ETS-Lindgren HI-3603 VLF Survey Meter - Manual (Page 27)

HI-3603 VDT/VLF Survey Meter
© ETS-Lindgren, August, 2005
Revision E, Part #H-600042 27
harmonics. See Figure 2 for an illustration of the frequency
response of the HI-3603.
In addition to the horizontal sweep-frequency circuit, there is a
vertical deflection circuit that 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 that are strongly
vertically polarized near the front of the screen. This aspect is
important when characterizing the magnetic fields of VDT's with
the HI-3603 (see the Characterizing VDT Emissions and
Operator Exposure section).
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-3603 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.