ETS-Lindgren HI-3627 ELF Magnetic Field Meter - Manual (Page 29)

HI-3627 Manual Page --21
7.0 Principle of Operation
The circuitry of the HI-3627 can be divided into six sections;
the probe, input signal conditioning, filter section, RMS
calculation, information processing and power supply. Each of
these is discussed in the follow sections (see Figure 7-1).
Probe
The probe consists of three concentric shielded coils aligned
with their axes perpendicular to one another. The electrostatic
shield of each coil is isolated from that of the other. The shields
are connected together at a single point at the base of the
sphere defined by the coils. The signals from each coil are
brought to the meter through a shielded twisted pair cable with
its shield connected to the coil shield common point.
Input Signal Conditioning
The output voltage of a coil varies with the frequency of the
applied magnetic field. A single pole low pass filter is added to
the output of each coil to flatten the frequency response and
provide impedance matching. The input signals are then passed
through a programmable gain amplifier stage that provides the
necessary amplification for the individual ranges. The signals
then pass to the filter section.
Filter Section
Each channel is filtered separately. The first stage is a two pole
Butterworth low pass filter at 2000 Hz. A Butterworth filter
attenuates the signal by 3 dB at the cutoff frequency with
attenuation in the stop band and no ripple in the pass band. The
output of the low pass filter is fed into two 4 pole high pass
filters. The cutoff frequencies on these filters are 5 Hz and 30
Hz corresponding to the low frequency cutoff selections.