Sect. 11 Page 6
If F = random average frequency, b. Place the input signal across the INPUT ter-
and fi = indicated frequency, minals.
and fs = frequency represented by full
scale in use, c. Note the reading on the meter and step down
and d = width of current pulse the RANGE switch as necessary to obtain a satis-
x number pulses, factory meter reading. As the RANGE switch is
then d = .6O/fs(fi). stepped down, fs in equations (l), (2), and (3)
is reduced, and the ?predictable error? correction
The time available for counting an input pulse increases. However, the basic meter accuracy -
is given by the expression: an unpredictable quantity - improves because the
meter pointer indicates further upscale as the
Time avail. 2 1-d or 1 - .60 f /f RANGE switch is stepped down.
is
The random average frequency with no expansion d. When the indicated reading has been obtained
is: apply the applicable correction factor from equa-
tion (2) for XI0 expansion, or equation (3) for
Fg f i X3 expansion.
l-.6fi/fs
With X10 expansion:
- EXAMPLE -
With the instrument set up as described in steps
a. and b. above, a measurement is taken and the
With X3 expansion:
RANGE switch is stepped down to the 10 KC posi-
FZ f i tion. This action places the X10 marking in the
(3)
1- .2fi/fs RANGE switch dial skirt at 1 KC. The meter reads
4.5. Therefore, it represents an uncorrected aver-
age count of 450 cps. Applying equation (2) for a
2B-8 RANDOM COUNTING PROCEDURE
random average count we have:
a. Place controls in the following positions:
F= f i
1 - .06fi/fs 7
RANGE - 100KC
EXPAND - XI0
- 450
OFFSET - both controls to zero, full
clockwise. 1- .06(450)/1000
= 450/.973
Under these conditions the zero position of the
OFFSET control calibrates zero on the meter (0-10
= 462 cps(*l scale division)
scale) so that an absolute reading may be obtained
on the expanded scale from zero to 10 KC. The
10 KC range is not used because, as explained
in paragraph 2B-7C, 60 percent of the period time
of a basic range frequency is used to develop the 2B-9 OPERATING CALIBRATION AND CHECK
current pulse and is not available for counting.
Less time is used developing the shorter current Two performance checks are available for the
pulses for the 100 KC range than is used for those operator in checking the accuracy of the instrument
developed on the 10 KC range. This means that by during operation. Perform CAL check first. If
expanding you achieve a reading which is closer two checks cannot be made to agree, seeparagraph
to the actual random count, since the probable counts 4-10.
missed during the development of the constant cur-
rent pulses through the meter will be fewer. Thus CALIBRATION (CAL)
the correction which is applied as the ?predictable This check permits amplitude of the pulse fromthe
error? will be smaller in magnitude. However, un- constant current source in the meter to be cali-
expanded ranges may be used. The disadvantage brated. The procedure is as follows:
is that the readings will be further from the actual
count, and the ?predictable error? correction will a. Turn on the instrument and permit it to reach
be correspondingly larger. a stable operating temperature. |
ACEC
