© National Instruments
|
3-15
•
CI.Freq.EnableAveraging—Set this property to False.
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 signals used as the counter timebase, set this property
to a different terminal.
Low Frequency with 1 Counter
For this method, the device detects the first full period of the signal-to-measure. The counter
measures the duration of this period by counting the number of cycles (
T2
) of the timebase.
DAQmx reads
T2
, and uses the known frequency of the timebase (
f
k
)
, to calculate the frequency
of the input signal (
f
x
) as:
Figure 3-17.
Frequency Measurement: Low Frequency with One Counter
The maximum error and maximum frequency error for this method, are given by:
To use the Low Frequency with 1 Counter method, configure the following:
•
DAQmx Create channel (CI-Frequency)—Use this VI or function to create the channel.
•
CI.Freq.MeasMeth—Set this property to Low Frequency with 1 Counter.
f
x
f
k
T
2
------
=
1
2
3
…
T2
…
Fre
qu
ency of
f
=
T2
Interv
a
l Me
asu
red
f
DAQmx
S
t
a
rt T
as
k
x
k
S
ign
a
l to
Me
asu
re (f )
Co
u
nter
Time
bas
e (f )
x
k
Maximum Error (%)
f
x
f
k
f
x
–
-------------
=
100%
×
Maximum Frequency Error (Hz)
f
x
f
x
f
k
f
x
–
-------------
×
=