5-16
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Chapter 5
Counters
Which Method Is Best?
This depends on the frequency to be measured, the rate at which you want to monitor the
frequency and the accuracy you desire. Take for example, measuring a 50 kHz signal. Assuming
that the measurement times for the sample clocked (with averaging) and two counter frequency
measurements are configured the same, Table 5-3 summarizes the results.
Table 5-2.
Frequency Measurement Methods
Variable
Sample Clocked
One Counter
Two Counter
High
Frequency
Large Range
fk
Known timebase
Known
timebase
Known timebase
Measurement
time
gating period
Max.
frequency
error
fk
Max. error %
Note
: Accuracy equations do not take clock stability into account. Refer to the specifications document
for your cDAQ chassis for information about clock stability.
Table 5-3.
50 kHz Frequency Measurement Methods
Variable
Sample
Clocked
One Counter
Two Counter
High
Frequency
Large Range
fx
50,000
50,000
50,000
50,000
fk
80 M
80 M
1,000
80 M
Measurement
time
(
mS
)
1
.02
1
1
N
—
—
—
50
1
gating period
-------------------------------
1
fs
----
1
fx
----
N
fx
----
fx
fx
fk
fx
fs
----
1
–
×
-------------------------------
×
fx
fx
fk fx
–
---------------
×
fx
fx
N fk fx
–
×
-------------------------
×
fx
fk
fx
fs
----
1
–
×
-------------------------------
fx
fk fx
–
---------------
fk
fx
----
fx
N fk fx
–
×
-------------------------
