Arbiter Systems 931A, Руководство по эксплуатации

Страница: 24 / 161

2.4.1 Sampling and Filtering
The Model 931A normally samples input signals at approximately 62.5
thousand samples per second (kS/s) and employs a 15 kHz, two-pole
analog anti-alias filter. The signals are filtered again using a digital low-
pass filter to remove signals outside the measurement bandwidth (3.05
kHz). The digital filter provides greater accuracy and repeatability and
a better rejection rate than could be accomplished with analog filtering.
Measurements performed with this technique are essentially free of the
harmful effects of aliasing.
8,192 samples are taken from Channels 1 and 2 during each mea-
surement cycle. This process requires approximately 131 milliseconds.
Depending on the exact measurements made, approximately three mea-
surement cycles are executed per second.
Since substantial dead time
between measurements occur, the Model 931A is not capable of performing
cycle-by-cycle, real-time monitoring.
For fundamental signals below approximately 40 Hz, the sampling rate
is automatically reduced to 12.5 Ks/s, allowing accurate measurements at
frequencies down to 5 Hz. The record length under these conditions is 655
ms.
The Model 931A completes a measurement cycle 2 to 3 times per
second. The cycle includes collecting data, processing and displaying
results. A faster update rate of 4 to 5 times per second is achieved by
initiating the “Fast Mode.” Pressing the shift key twice, or issuing the SPD
RS-232 command, starts this mode. The speed increase is accomplished
by eliminating the computation of harmonic data. If the unit is in the Fast
Mode and the harmonics display is selected it will revert back to normal
operation. Using the Fast Mode does not degrade the accuracy of the
Model 931A.
2.5 User Interface
The user interface for the Model 931A is designed to streamline overall
operation by making the most commonly used functions easily accessible.
The following paragraphs briefly describe the Model 931A user interface.
2.5.1 Graphics Display
A supertwist liquid crystal display (LCD) serves as the main display
and is capable of displaying both characters and graphics, including the
waveforms of the input signals. The display features a backlight that can
be enabled for operation in subdued lighting conditions.
8