Detailed Description
8500C/8500C+ System Maintenance Manual
4-5
4.2.1.1.2 Digital Board (Figure 4-3)
The digital board receives digital signals from the ADC on the analog board and digital serial
communications from the microcontroller on the controller board in the Signal Selector 8520C. The
signals from the analog board are analyzed and processed by the digital board, and the results are
displayed on the liquid crystal display (LCD) or printed out by the printer. The serial communications
from the 8520C are used to select the Velocimeter or magnetic pickup inputs. The analog board also
sends detected overload digital signals to the digital board. The overload signals are analyzed by the
digital board and control signals are sent to the analog board to control amplifier gain. The digital
board also uses the data retrieved from disk and the user’s keypanel input to analyze and process the
input data from the analog board.
a.
Pulse Detector. The pulse detector is a threshold adaptive detector which is used to
translate the azimuth pickup input signal from the 8520C to digital logic levels. The
detector detects peak pulse levels as low as 0.5 volt.
b.
Timing Controller. The timing controller is a monolithic, five-channel, high resolution
controller which is used to manage all data acquisition timing. The controller contains a 5
MHz crystal time base to provide stable time measurement with a resolution of
200
nanoseconds.
Four timing channels of the controller are used in various firmware programmed
configurations for vibration data sampling and azimuth pickup period measurement. The
fifth channel is used to generate a precise clock for the switched capacitor filters on the
analog board.
For measurements at frequencies up to 500 Hz, a counter/timer controls sample rates with
a worst-case resolution of one part in
2250
. Above 500 Hz, the worst-case resolution is
one part in 196.
c.
Communications Controller. The communications controller is a dualchannel,
asynchronous serial communications controller which is used to interface with the 8520C
and the printer. The controller includes a general purpose parallel interface which is used
to control both the programmable gain amplifier on the analog board and the realtime
clock.
d.
Realtime Clock. The realtime clock is a battery sustained realtime clock which maintains
the system time, day, and date. The battery is only used when external power is not applied
to the 8500C/C+.
e.
Digital Filters. The digital filters consist of decimating finite impulse response (FIR)
filters that further low-pass filter the digitized signal from the analog board. The filters
also provide sample rate conversion so that analysis at arbitrary frequencies is possible.
The low-pass filter characteristic achieved with the decimating FIR is nearly ideal, which
provides more unaliased
bins
after time-to-frequency domain conversion, 15% more than
usual.
The firmware configures the digital anti-alias filter with from one to eight stages. The
sample rate may thus be reduced by a factor as large as 256.
Amplitude response through all stages is flat within 0.05 dB. The phase response through
each stage is linearly dependent on frequency relative to sample frequency. By virtue of
their digital implementation, component values, tolerances, and temperature do not affect
the response of these filters.
