3-14
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Chapter 3
Counter Input
To use the Sample Clock (with Averaging) method, configure the following:
•
DAQmx Create channel (CI-Frequency)—Use this VI or function to create the channel.
•
DAQmx Timing (Sample Clock)—Use this VI or function to set the number of samples,
sample clock source, and other properties.
•
CI.Freq.EnableAveraging—Set this property to True.
By default, the counter measures the frequency on a default PFI terminal (refer to Chapter 5,
, for more information) and use an onboard 100 MHz clock as the
timebase. To change the signals used for this measurement, configure the following:
•
CI.Freq.Term—The signal-to-measure comes from an input terminal. To change the
signal-to-measure, set this property to a different terminal.
•
CI.CtrTimebase.Src—To change the signal used as the counter timebase, set this property
to a different terminal.
Sample Clock (without Averaging)
With this method, for each sample clock the counter detects the last full period of the
signal-to-measure that occurs before the sample clock. The counter measures the duration of this
one period by counting the number of cycles (
T2)
of the timebase.
At each sample clock, the device stores
T2
in a buffer. DAQmx reads this value, and uses the
known frequency of the timebase (
f
k
), to calculate the frequency of the input signal (
f
x
) as:
Figure 3-16.
Frequency Measurement: Sample Clock without Averaging
To use the Sample Clock (without Averaging) method, configure the following:
•
DAQmx Create channel (CI-Frequency)—Use this VI or function to create the channel.
•
DAQmx Timing (Sample Clock)—Use this VI or function to set the number of samples,
samples clock source, and other properties.
f
x
f
k
1
T
2
------
×
=
Sa
mple
Clock
DAQmx
S
t
a
rt T
as
k
B
u
ffer
6
6
4
6
4
6
4
6
6
Sa
mple 1
Sa
mple 2
Sa
mple
3
S
ign
a
l to
Me
asu
re (f )
Co
u
nter
Time
bas
e (f )
x
k
