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8
Theory of Operation
16902A/16903A Front Panel Assembly
114
of a programmable 12 bit DAC and comparator. This
combined with a level shift and divide circuit allows for a ±5
V input range, 200 mV minimum swing, and 50 mV
threshold steps.
Clock IN
This BNC input is specifically for a 10 MHz clock signal from
an oscilloscope or other source. The signal is AC coupled
and passed through a 10X multiplier part (low jitter) and
then fed to the FPGA as one of the 100 MHz clock source
choices. Using this input allows two measurement
instruments to run on the same time base and maintain
correct time alignment over long aquisitions.
Calibration OUT
This BNC output is a low frequency, 49 KHz, signal driven
from a LVTTL 50 Ω line driver. It is used by an external
Time Correlation Fixture to simultaneously trigger multiple
frames and oscilloscopes. This is then used with a software
averaging program to closely correlate the trigger point
across instruments.
The 16900A frame does not have a liquid crystal display
(LCD). The identical front panel assembly with LCD is used
on both the 16902A six- slot logic analyzer and the 16903A
three- slot logic analyzer. The front panel assembly is
composed of two PC boards comprising three independently
operating sub- systems. The two PC boards are the front
panel interface board and the keypad board. The three front
panel sub- systems are the front panel keyboard, front panel
LCD display, and the front panel touch screen.
The keypad board is controlled by a programmable
microcontroller, the Intel 87C51. The ports of this
microcontroller are connected to a matrix of push buttons,
digital knobs, and LEDs. The microcontroller will scan the
buttons and knobs looking for button presses and releases
and knob rotations. The microcontroller contains a +5 V
RS232 interface, which is used as the communication
interface to the host. The logic analyzer software will send
command codes to this microcontroller to receive button and
knob status and to turn on and off individual LEDs. The
microcontroller part used is a program- once part which is
16900A, 16902A, and 16903A Service Guide
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Troubleshooting
