ETS-Lindgren HI-3627 ELF Magnetic Field Meter - Manual (Page 31)

HI-3627 Manual Page --23
The front panel switch controls an analog multiplexer which
selects the proper filter output.
RMS Circuitry
The signal from each filter is fed into one of three true RMS
converter integrated circuits, one for each channel. The time
response of the RMS converter is selected to give acceptable
step input response time while yielding minimal RMS conversion
errors at 5 Hz. This time constant is around 500 msec. This is
the predominant time constant of the meter.
Information Processing
The outputs of the three true RMS converters are combined
through vector addition to calculate the resultant field
magnitude. This computation is computed by a processor and an
A/D converter. The meter is driven by outputs configured as a
PWM (Pulse Width Modulator). The processor performs an A/D
conversion on the signal from each axis by programming an
analog multiplexer into a 12 bit A/D converter. The digital values
are then squared, summed and the square root of the sum is
calculated. This result is used to set the duty cycle of the PWM
output which drives the meter movement. The output frequency
of the PWM is about 30 Hz. This frequency is too high to show
on the analog meter, but can be detected as about 50 mV of
ripple on the recorder output.
Power supply
Power is supplied to the HI-3627 by a NiCad battery. The NiCad
battery chosen for this application is a common type. The
current consumption of the instrument is relatively low with the
operating life being greater than 30 hours of continuous
operation. During typical daily operation, with intermittent use,
the time between recharges will be greater. The individual
voltages supplied to the circuitry are derived from the battery
voltage directly, supplied through series pass regulators and
from a switched capacitor voltage invertor.