© National Instruments
|
3-13
Frequency Measurement Methods
This section describes the frequency measurement methods supported by the NI 6614.
Sample Clock (with Averaging)
With this method, for each sample clock the counter counts the number of full periods (
T1
) of
the signal-to-measure since the previous sample clock. It also measures the duration of these full
periods by counting the number of cycles (
T2)
of the timebase.
Figure 3-15.
Frequency Measurement: Sample Clock with Averaging
At each sample clock, the device stores
T1
and
T2
in a buffer. DAQmx reads these values, and
uses the known frequency of the timebase (
f
k
), to calculate the frequency of the input signal
(
f
x
) as:
The maximum error and maximum frequency error for this method, are given by:
S
ign
a
l to
Me
asu
re (f )
Co
u
nter
Time
bas
e (f )
Sa
mple
Clock (f )
DAQmx
S
t
a
rt T
as
k
T1 T2
1 7
T1 T2
1 7
2 10
T1 T2
1 7
2 10
1 6
B
u
ffer
1
2
1
7
10
6
Sa
mple 2
Sa
mple
3
Sa
mple 1
x
k
s
f
x
f
k
T
1
T
2
------
×
=
Maximum Error (%)
f
x
f
k
f
x
f
s
---
1
–
×
----------------------------
=
100%
×
Maximum Frequency Error (Hz)
f
x
f
x
f
k
f
x
f
s
---
1
–
×
----------------------------
×
=
