4-5
The third protection mechanism to protect devices from damage is not shown on the
diagram but is part of the control logic circuit. This is the requirement that two push-buttons
on the front panel must be pressed at the same time for any relay to actuate. These would be
either the A or B line button and any of the 32 relay selector push-buttons. This is to help
prevent accidental operation. It takes two hands to operate the scanner from the front panel.
4.6 PROCEDURE FOR MEASURING VOLTAGE REFERENCES
The National Institute of Standards and Technology has recommended a method of
inter-comparing voltage reference standards by measuring the difference between the units in
pairs in a statistically balanced design. Usually the number of unit pair measurements is twice
the number of units being compared. One possible design for comparing two groups of four
units is shown in Figure 4.5.
READING
A LINE
B LINE
El
Group
1, Unit 1
- Group
2, Unit 1
E2
Group
1, Unit 1
- Group
2, Unit 3
E3
Group
1, Unit 3
- Group
2, Unit 3
E4
Group
1, Unit 3
- Group
2, Unit 1
E5
Group
1, Unit 2
- Group
2, Unit 2
E6
Group
1, Unit 2
- Group
2, Unit 4
E7
Group
1, Unit 4
- Group
2, Unit 4
E8
Group
1, Unit 4
- Group
2, Unit 2
E9
Group
2, Unit 2
- Group
1, Unit 1
E10
Group
2, Unit 2
- Group
1, Unit 3
Ell
Group
2, Unit 4
- Group
1, Unit 3
E12
Group
2, Unit 4
- Group
1, Unit 1
E13
Group
2, Unit 1
- Group
1, Unit 2
E14
Group
2, Unit 1
- Group
1, Unit 4
E15
Group
2, Unit 3
- Group
1, Unit 4
E16
Group
2, Unit 3
- Group
1, Unit 2
FIGURE 4.5: 4x4 MEASUREMENT DESIGN
As can be seen in the figure the eight units are compared using 16 measurements. Note
that each unit appears on the A side twice and on the B side twice.
When all the unit difference measurements are complete, a least squares fit of the data
can be computed. The estimated, or least squares optimized, differences can be compared to the
measured values to compute a standard deviation characterizing the random error of the
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