ETS-Lindgren HI-3637 VLF Magnetic Field Meter User Manual

Page 31

3637 Manual

Page — 23

7.0 THEORY OF OPERATION

Introduction

The circuitry of the HI

3637 can be divided into seven

sections: probe, range select, pre-filter, integrator, post-

filter, RMS calculation and the power supply. Each of

these is discussed in the follow sections (see Figure

1).

Probe

The probe consists of three orthogonal concentric

shielded coils aligned such that their axes are mutually

perpendicular. Each coil's electrostatic shield is isolated

from that of the others: the shields are connected at a

single point at the base of the sphere defined by the

coils. The signal from each coil is routed to the meter

through a shielded twisted-pair cable: this cable's shield

is also connected to the common point of the coil

electrostatic shields.

Range Select

The output voltage of a coil varies with the frequency of

the applied magnetic field. In order to accommodate the

magnetic field amplitudes and frequencies that the

3637 is designed to measure, a set of selectable

attenuators is used to ensure that the input voltage to the

remaining circuitry is at the proper level for the selected

range.

Pre-filter Section

Each channel is filtered separately. A fourth-order high-

pass Butterworth filter is used to insure that only signals

with a frequency of 2000 Hz or greater reach the

integrator. The Butterworth filter attenuates the signal by

3 dB at the cutoff frequency, has maximum attenuation

in the stop band, and has no ripple in the passband.