ETS-Lindgren HI-3627, User Manual

Page: 31 / 39

ets-lindgren.com Principle of Operation 31
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.