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display's least significant digit, which is present in all
electronic counters. This error is due to phasing be¬
tween the timing pulse that operates the electronic
gate and the pulses that pass through the gate to the
counting assembly. The chart in Figure 3-3 shows the
error possible for frequency and period measurements.
3-29.
The formula for determining the actual fre¬
quency is given as follows:
error = ± (-i-) ± E
fj x gate length (sec)
The expression
j
^ X gate length (sec)
equals the ± 1 count ambiguity, where f| equals meas¬
ured frequency (Hz) and gate length
equals the
selected gate time in seconds. E equals the time base
accuracy (monthly drift rate of the individual time
base times the number of months since calibration,
frequency
change
due
to
ambient
temperature
change, absolute off-set at standardization, and line
voltage effects).
3-30. An example of frequency error calculation is as
follows:
fx = 3 MHz (3 X 106 Hz)
gate length = .1 sec (1 X 10'1)
E = 3 parts in 107 per month times 2 months
= 6 parts in 107
error =-*-
(3 X 106) (1 X 10-1)
107
= 3.3 X 10-6 ± 6 X lO"7 = 3.9 X 10~6
or 3.9 parts in 10c
Model 5326/27A
Operation
3-31.
PERIOD MEASUREMENTS.
There are three
factors contributing to the accuracy of period average
measurements:
a. The aging rate of the 10 MHz crystal standard.
b.
The ± 1 count ambiguity.
c.
The trigger error for one period.
Assuming a signal-to-noise ratio of 40 dR, the
trigger error is less than 0.3% at rate sensi¬
tivity. A general formula for finding the per¬
centage error to be expected under various
conditions is as follows:
f
A =100 (±
2_± —§—± E)
nf
n
A = Accuracy in percent
f^ = Time base frequency counted
fg = Frequency of input signal (Hz)
n = Number of periods averaged
e = 3 x 10 3 (trigger error for one period,
40 dB S/N at rated sensitivity.)
E = time base accuracy (monthy drift
rate of individual time base times the
number of months since calibration,
absolute value of off-set at standard¬
ization,
frequency change
due
to
ambient
temperature
change,
and
line voltage effects).
A plot of the
above formula is shown in Figure 3-3.
3-3
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