17
TROUBLESHOOTING
To start, make sure that the power entry module
on the rear panel is set for ac line voltage in
your area, and that the correct fuse is installed.
The programmed voltage may be seen through a
window when the power entry module is viewed
from the bottom of the unit. Verify that the line
cord is plugged all the way into the power entry
module, and that the power button on the front
panel is pressed "in".
When the ac power is applied, you should be
able to hear the fan: the unit will not function
properly if the fan is not operating or if the side
or bottom vent holes are blocked.
Set the rear panel switch "down" to select the
INT timebase. If this switch is "up", and a
reference is not applied, then the error message
"Ext Clk Error" will appear on the LCD.
LCD CONTRAST
If there are no characters on the LCD, or the
contrast is very poor, adjust the contrast with the
two right most keys (the ones with the gray
up/down arrows). The up arrow will increase the
contrast, the down arrow will decrease the
contrast when the keypad is in the cursor mode.
COLD BOOT
If the instrument turns on, but is completely
unresponsive to the keyboard, then the RAM
contents may have been corrupted causing the
instrument to "hang". To remedy this situation,
turn the unit off, then hold down the BSP
(backspace) key down and turn the unit back on
again. This procedure initializes the RAM. User
calibration parameters will be overwritten by the
factory calibration parameters and GPIB address
will be set to 15.
QUICK TEST
Unplug all cables from the unit and recall the
"default" settings by pressing the "RECALL"
menu key, the "0" key, and the "EXC" key in
sequence. The trigger menu will appear on the
LCD with the cursor under "Ss" for single shot.
Press "TRIG" to select this mode. To trigger the
unit once, press the "EXC" key. The TRIG and
BUSY LED's on the left side of the instrument
will blink once each time the "EXC" key is pressed.
Now press the left arrow key (the "4" key) twice to
select the Internal trigger source. The default trigger
rate is 10 kHz, so the TRIG and BUSY LED's will
now glow steadily. Trigger an oscilloscope on the
rising edge of the T0 output, and use the scope to
look at the output from channel A on the 1µs per
division scale.
The default time delays are all zero. Press the
DELAY Menu Key to show the A delay setting.
Use the left/right cursor keys to position the cursor
beneath the 1µs digit (seventh from the right). Use
the up cursor key to increment the delay in 1µs
increments. The rising edge of the A output will
move 1µs later each time the "up" cursor key is
pressed. The RATE error LED will come on if the
delay setting exceeds 99µs, as the trigger period is
100µs (10kHz) and 1µs is required for the reset
cycle. If channel A is set for a longer delay than any
of the other channels, its pulse will have a constant
pulse width of about 800ns.
OUTPUT LEVELS
If an output has only half of the programmed
amplitude, then it is very likely that a high
impedance load was specified in the OUTPUT
Menu, but a 50
Ω
load is attached. If an output
behaves very erratically, then it is very likely that a
50
Ω
load was specified, but a high impedance load
is attached. The pulse outputs, AB and CD, will
misbehave if a 50
Ω
load is specified for these
outputs, but a 50
Ω
load is attached to only one side
of the pair (i.e., to the AB output but not to the -AB
output).
JITTER
The most common causes of excess jitter are (1)
incorrect external trigger threshold setting, (2) noise
or amplitude fluctuations on the trigger input, (3)
insufficient or excessive trigger amplitude, (4)
blocked or stalled cooling fan, or (5) triggering at
too high a rate. The instrument is specified to have
a jitter of 50ps + 0.01ppm of the delay (rms).
The
peak-to-peak jitter, as seen on an oscilloscope, is
approximately four times the rms jitter, hence one
would expect to see about 200ps peak-to-peak on an
oscilloscope for short delays.
