Racal Instruments 2251A Operator's Manual page 83

4.6 Calibration, External Standard Output (2251 with option 11)
4.6.1 Internal Frequency Standard Adjustment
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Prior to calibration, the 2251 or 2251A must be in continuous
operation for 4 hours at an ambient temperature of 25 +/-2 C
Prior to calibration of the 2251, 2251A (with Option 11) place the
module in a chassis which allows access to the frequency adjustment
screw (Racal 1264C Chassis) or place the module on an extender card.
Connect the master frequency standard (9480/FRS or equivalent)
to the EXT STD INPUT of the 2251 Frequency Counter.
Connect the 2251A 10MHz (UUT) reference output to INPUT of the
2251A.
Figure 4-19, External Standard Output
Program the 2251 Frequency Counter as follows:
1. Frequency, Channel A
2. AC coupled 50 ohm input impedance
3. Auto-Trigger
4. External Reference Input
5. 10 S Gate Time
6. Enable math function for deviation read-out.
Use a screw driver to adjust the Frequency Adjust Screw until a
reading of +10.000000000e+06 or FA10MHz is reached.
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