ETS-Lindgren HI-3637 VLF Magnetic Field Meter - Manual (Page 41)

HI n 3637 Manual Page — 33
60/50 Hz Fields
These fields are caused primarily by the current flowing
in the vertical deflection coil and are nearly symmetrical
around the coil. It is produced by the same mechanism
that produces the DC field; the charge on the VDT screen
which produces the DC field is actually not constant but
builds up and decays by a small amount each time the
display is scanned by the electron beam. This occurs at
a nominal 60 Hz rate although harmonics may exist up to
several kHz (Harvey, 1983a). Measurements of the 60 Hz
emissions and harmonics by Stuchly, et al, (1983) found
magnetic field strengths of 100 to 200 mA/m at a
position 30 cm in front of the VDT. Harvey (1984b)
reported measured 60 Hz AC electric field strengths of
between 5 and 60 V/m in an investigation of five VDTs.
These relatively low values are in the range of other
commonly encountered 60 Hz appliances found in the
home and office environment.
Deflection System Fields
The principal RF component of VDT emissions is caused
by the so-called flyback transformer circuitry which is
responsible for a rapidly changing current which flows in
the horizontal deflection coils of the VDT and causes the
electron beam to be rapidly swept to the left side of the
screen, ready for another trace across the screen. The
rate at which the electron beam is scanned is dependent
on the particular design of the VDT but typically falls in
the range of 17 to 30 kHz. For television receivers, the
flyback frequency is approximately 15.75 kHz.
The flyback circuit is rich in harmonic production and any
instrument intended for accurate assessment of RF
exposure fields produced by VDT's must be capable of
true RMS measurement. The strong harmonic content of
the flyback signal means that it has a non-sinusoidal
waveform; the HI n 3637 incorporates a true RMS
detector circuit which can accurately respond to the
complex waveforms observed near VDT's.
Approximately 95 percent of the total energy of the
flyback circuit emissions is contained within the first five
or six harmonics. Consequently, the bandpass of the HI-
3637 has been tailored to the necessary frequency range