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Maintenance— 2440 Service
M anual A djustm ents
Adjustments requiring internal access to the scope are
limited to the display system, CCD output amplifier gains,
input
capacitance,
100 MHz
bandwidth
limiter
(and
20 MHz bandwidth limiter with the Video Option), CCD
clocks, and transient response. These adjustments are
made during factory calibration and should not require
readjustment during normal operation. Replacement of
parts during repair of the instrument that affect these cali
brations will, however, require readjustment of the affected
circuitry.
The
ADJUSTS
calibration
routine
in
the
EXTENDED CALIBRATION procedures provides display
patterns and brief instructions for the technician to follow
in calibrating the display system and CCD output
amplifier gains.
D ynam ic A djustm ents
As the instrument operates, it continuously corrects for
minor offsets in the acquisition system and jitter correction
ramp timing. These “dynamic" adjustments are totally
automatic and require no user action.
R eco m m end ed A d ju stm ent Intervals
Perform the ATTEN and TRIGGER parts of the
Extended Calibration procedure every 2000 hours, or once
a year if the instrument is used infrequently. Readjustment
of the Display System and rerunning the REPET calibra
tion step is not normally needed unless parts are replaced
that affect those calibrations. It is NOT necessary to reper
form any portion of the Extended Calibration to maintain
maximum measurement accuracy over the specified
operating temperature range of the instrument.
NATIONAL BUREAU OF STANDARDS
TRACEABILITY
Traceability to the National Bureau of Standards (NBS)
requires that the stated accuracy of an instrument be
established by calibration with equipment whose accura
cies have been directly or indirectly established by NBS
certified references.
For this oscilloscope, traceability is established in the
Extended Calibration routine by calibrating the attenuators
(ATTEN) and external trigger amplifiers (TRIGGER) with an
NBS traceable voltage reference. As the fine gain adjust
ment of the attenuators is made, the relative accuracy of
the internal 10-V Calibration Reference is also checked by
normalizing it to the external voltage source provided by
the technician. If the fine gain of the attenuators requires
an adjustment of more than approximately 2%, the ATTEN
calibration fails. Barring internal component problems, a
failure indicates either that the internal reference is faulty
or that the applied voltage is not a valid standard refer
ence voltage.
Passing the ATTEN calibration step using an NBS
traceable voltage standard ensures that the internal,
nonadjustable 10-V Calibration Reference is also trace
able. Subsequently passing the SELF CAL procedure
(which uses the traceable 10-V Calibration Reference to
provide the calibration voltages) then makes this scope an
NBS traceable instrument. Traceability is maintained for
subsequent performances of SELF CAL by referencing all
calibration
calculations
to
the
traceable
internal
voltage reference.
VOIDING CALIBRATION
Factory calibration of this scope is done using NBS
traceable sources. An internal jumper installed at the time
of calibration prevents the user from inadvertently running
the EXT CAL routines and voiding the traceable calibration
of the instrument. Removing the jumper and attempting to
do the ATTEN and TRIGGER calibration without an accu
rate standard amplitude voltage source will result in a
failed calibration. In the case of a failure, the stored con
stants for the attenuator gain calibration are not replaced;
therefore, the previous degree of accuracy is maintained
by the instrument.
However, a FAIL label remains
displayed over the affected EXT CAL menu choice, and
the scope will fail subsequent power-on tests and enter
Extended Diagnostics (EXT DIAG) until the calibration
is passed.
Power-on or Self Diagnostics (SELF DIAG) tests that
detect a system, subsystem, or device failure that may
affect instrument calibration are noted by a FAIL label on
the test along with the calibration status of UNCALD in
the EXT DIAG menu display. Calibration failures are of two
types: soft errors caused by gain or offset parameter drifts
beyond tolerance— usually caused by a large change in
6-2
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