ETS-Lindgren HI-3603 VLF Survey Meter - Manual (Page 26)

HI-3603 VDT/VLF Survey Meter
© ETS-Lindgren, August, 2005
Revision E, Part # H-600042
26
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 that 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 5 VDT's. 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-3603
incorporates a true RMS detector circuit that 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-3603 has been tailored to
the necessary frequency range to capture all the important