3-12
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Chapter 3
Counter Input
In both cases, the frequency,
f
, is given by:
Figure 3-13.
Frequency Measurement Using Time
Figure 3-14.
Frequency Measurement Using Periods
Trade-offs
Consider the following trade-offs when determining the method of measuring frequency:
•
Accuracy vs. Update Rate
Increasing measurement duration increases the accuracy of the measurement.
Decreasing measurement duration allows the counter to update the measurement more
often. If the input frequency changes often, it may be better to take many less-accurate
measurements than a few more accurate ones.
•
Sample Clock vs. Implicit Timing
If you use a sample clock, the device returns one frequency measurement for each
sample clock.
If you use implicit timing and specify a measurement time, the device returns the
measurement after that amount of time.
If you use implicit timing and specify a number of periods to measure, the device returns
the measurement after that number of periods of the input signal. Note that the time to
return a measurement depends on the (unknown) frequency of the input signal. If the input
signal is at a low frequency, the device will take a long time to return a measurement.
•
Number of Counters
Some methods use two of the eight counters on the NI 6614; other methods use one of the
eight counters.
f
p
t
---
=
Co
u
nt
S
ign
a
l to Me
asu
re
Co
u
nter Time
bas
e
(100 MHz)
me
asu
rement d
u
r
a
tion = 100 µ
s
1
0
2
50
Co
u
nt
S
ign
a
l to Me
asu
re
Co
u
nter Time
bas
e
(100 MHz)
me
asu
rement d
u
r
a
tion
=
3
period
s
1
4
3
5
6
2
