Chapter 6 – Maintenance
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For more information on the display, see Section 3.2.1
“Display”.
6.2.3 Analog Subsystem
The primary purpose of the analog subsystem is to route,
condition, and filter the velocity vibration and azimuth
(photocell system or Model 3030 Magnetic Pickup) signals. This
subsystem also performs some self- calibration functions.
6.2.3.1
Velocity Signal
The Vibrex unit routes velocity vibration signals from one of two
channels by a multiplexer (also called a mux). The unit offsets
these signals to 2.5V, then passes them through a two-pole high-
pass filter having a corner at 4 Hz. The signals then pass through
a six-pole, low- pass antialiasing filter with a corner frequency at
380 Hz.
These conditioned signals then go to a 13-bit ADC. The 13 bits
include
12 bits to signify the number plus one for the sign (+ or -). A
timing processor unit (TPU) in the microprocessor controls the
sampling. A hardware state machine that uses discrete logic
coordinates the data exchange between the microprocessor and
the ADC.
6.2.3.2
Azimuth Signal
The Vibrex unit routes azimuth (frequency or tachometer)
signals from one of two channels by a multiplexer. These signals
then go to a pulse detector, a threshold-adaptive detector that
translates azimuth pickup input signals (from the magnetic
pickup or photocell) to digital logic levels.
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