ETS-Lindgren HI-3638 ELV/VLF Electric Field Meter - Manual (Page 23)

www.ets-lindgren.com Application 23
6.0 Application
Meter
The HI-3638 ELF/VLF Electric Field Meter is a single axis meter (responsive to
one polarization component at a time) designed to be responsive to electric
fields, including non-sinusoidal waveforms, from 5 Hz to 400 kHz.
When the HI-3638 is immersed in an electric field, the sensor generates a
current. An input op-amp then generates a voltage which correlates to this
sensor current. This voltage is fed into a filter which isolates the ELF and VLF
ranges and discards the out of band signals. The output of the selected filter
stage, ELF or VLF, is passed to a software controlled gain stage with selectable
gains of x1, x10, x100, x1k, x10k. The output of the gain stage is converted to its
RMS DC equivalent which is then converted to a digital number. This digital
number is then converted to a displayable value and sent through fiber optic
cable to the HI-4416 Digital Readout/Control Unit.
Sensor Theory
Electric fields are measured through the employment of a displacement current
sensor
. A displacement current sensor operates on the principle that two parallel
conductive flat-plate electrodes, when electrically connected together, will exhibit
a displacement current which flows between the two plates when immersed in an
electric field. This can be visualized by remembering that the electric field
between two such plates must be zero when they are connected together;
because they are at the same potential there can be no electric field between
them (an electric field exists when the potential on the two electrodes is
different).
Another way of viewing this phenomenon is to understand that when immersed in
an electric field, the external field causes a redistribution of electric charge on the
two electrodes and this redistribution, or charge, is just flow of current, a
displacement current between the two plates.