Racal Instruments 2251A Operator's Manual page 25

Resolution (Number of Selected
Digits) in Frequency, Period,
Ratio, and Check (See Note 1)
9 + Overflow
9
8
7
6
5
4
3
When setting the resolution by programming the number of digits, use the following
formula to determine the value of the LSD (Least Significant Digit):
LSD = F x 10 where F = Frequency and D = Number of digits rounded up to the
next decade.
NOTE 1:
The most significant digit is permitted to exceed the resolution by 1 digit
providing a 10% overrange. This precludes unnecessary shifting of digits.
NOTE 2:
Measurements of frequency, period, ratio and check are averaged when these
gate times are set.
1.6.6.1 The resolution of phase and totalize is determined by the input signal.
1.6.6.2 Time interval, rise/fall time, and pulse width measurements have the resolution determined
by both the input signal and the resolution set.
1.6.6.3 Resolution is programmable from three to nine digits. But the minimum LSD the counter will
achieve is 1 ns, regardless of programmed gate time or resolution in time interval mode.
Exception is if measured time interval or period is $ 10 seconds.
1-16
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See Note 2
" "
" "
-D
Gate Time
10 S
1 S
100 ms
10 ms
1 ms
1 ms
1 ms
1 ms