Operator`s manual

SECTION 14. INSTALLATION AND MAINTENANCE
14-7
The following procedures are for calibrating the
voltage reference and the clock. Other factors
such as range ratios, DAC non-linearity, and
offset in either the switched excitation or the
CAO voltage require that the 21X be returned to
the factory for repair. Please call the factory to
obtain authorization before sending in the unit.
14.10.1 VOLTAGE REFERENCE CALIBRATION
PROCEDURE
SUGGESTED INSTRUMENTS
Five and one half digit digital volt meter (DVM)
with 10 microvolt resolution. The accuracy of
the DVM needs to be equal to that of the 21X
which is
±
1 mV at 5 V DC.
PROCEDURE
1. Remove the 21X battery base and unplug
the battery from the 21X. Remove the four
screws holding the aluminum cover plate
and remove the cover plate. Lay the 21X,
panel down, on a padded flat surface with
the circuit cards up and exposed as shown
in Figure 14.6-1. Plug the battery back into
the 21X. Turn on the 21X.
2. Set the DAC to output 0.000 volts DC by
programming the 21X as follows:
* 1 Table 1 Programs
01: 1 Sec. Execution Interval
01: P4 Excite,Delay,Volt(SE)
01: 1 Rep
02: 5 5000 mV slow Range
03: 1 IN Chan
04: 1 Excite all reps w/EXchan 1
05: 900 Delay (units .01sec)
06: 0000 mV Excitation
Change as instructed
07: 1 Loc [:MEAS. mV ]
08: 1 Mult
09: 0 Offset
3. Monitor the Digital to Analog Converter
(DAC) output by connecting the positive
lead of the DVM to the DAC OUTPUT
TEST POINT which is the left side of the
3.3 K resistor shown in Figure 14.10-1.
Connect the negative lead of the DVM to
any ground on the front panel. Set the DVM
to read on its most sensitive DC Volt scale.
4. Adjust the DAC OFFSET
POTENTIOMETER (see Figure 14.10-1) for
a DVM reading of 0.0000 V ±0.0001.
5. Change the 21X's program, so that
Instruction 4, parameter 6 reads 4000 to
change the DAC output to 4 V. Set up the
DVM to read 4 V DC.
6. Adjust the VOLTAGE REFERENCE GAIN
POTENTIOMETER (see Figure 14.10-1)
until a DVM reading of 4.0000 V ±0.0001 is
obtained.
7. Change the 21X's program, step 2, so that
Instruction 4, parameter 6 reads -4000 to
change the DAC output to -4 V.
8. A DVM reading of -4.000 V ±0.0025 verifies
the linearity. If this reading is off, the 21X
needs to be sent in for repair.
9. Lock both potentiometers into position with
a dab of finger nail polish.
14.10.2 CLOCK CALIBRATION PROCEDURE
The clock circuitry resides on the CPU card (the
card farthest from the front panel). The
frequency of the clock crystal exhibits a
parabolic response to temperature. The
frequency maximum occurs at room
temperature and drops off slowly at hotter or
colder temperatures. When the 21X leaves the
factory it is calibrated to be 20 ppm fast. If the
21X is placed in a controlled environment with
the temperature close to room temperature, it
would be better to set the crystal frequency
exactly on.
SUGGESTED INSTRUMENTS
Digital frequency counter
PROCEDURE