ETS-Lindgren HI-3637 VLF Magnetic Field Meter - Manual (Page 33)

HI n 3637 Manual Page — 25
Integrator Section
The output voltage of the probe sensor coils increases
directly with the frequency of the applied magnetic field.
The integrator provides a flat frequency response.
Post-filter Section
A second-order Butterworth low-pass filter ensures that
only signals below 400 kHz are fed to the RMS
calculation section.
RMS Calculation
The signal from each channel is applied to a separate true
RMS converter and combined through vector addition to
obtain the field magnitude. This value is computed by
the processor and A/D converter. The meter is driven by
outputs configured as a PWM (Pulse-Width Modulator).
The processor performs an A/D conversion on each axis
signal by programming the analog multiplexer, which
feeds the A/D converter. The digital values are squared
and summed; then the square root of the sum is
calculated. This result determines the duty cycle of the
PWM output, which drives the meter movement. The
PWM output frequency is 30 Hz; this is too high to be
visible on the analog meter, but it can be detected on the
recorder output as approximately 50 mV of ripple.
Power supply
A ten-cell NiCd battery powers the HI n 3637. The
current consumption of the instrument is relatively low,
yielding a continuous operating time of 20 hours. In
typical daily operation (intermittent use), the operating
time between charges will be much greater. The voltages
required by the analog circuitry are derived directly from
the battery voltage. The digital voltage is supplied via a
series pass regulator.