© National Instruments
|
5-13
Large Range of Frequencies with Two Counters
By using two counters, you can accurately measure a signal that might be high or low frequency.
This technique is called reciprocal frequency measurement. When measuring a large range of
frequencies with two counters, you generate a long pulse using the signal to measure. You then
measure the long pulse with a known timebase. The cDAQ controller can measure this long
pulse more accurately than the faster input signal.
Note
Counter 0 is always paired with Counter 1. Counter 2 is always paired with
Counter 3.
You can route the signal to measure to the Source input of Counter 0, as shown in Figure 5-14.
Assume this signal to measure has frequency
fx
. NI-DAQmx automatically configures Counter 0
to generate a single pulse that is the width of
N
periods of the source input signal.
Figure 5-14.
Large Range of Frequencies with Two Counters
Next, route the Counter 0 Internal Output signal to the Gate input of Counter 1. You can route a
signal of known frequency (
fk
) to the Counter 1 Source input. Configure Counter 1 to perform
a single pulse-width measurement. Suppose the result is that the pulse width is
J
periods of the
fk
clock.
From Counter 0, the length of the pulse is
N
/
fx
. From Counter 1, the length of the same pulse is
J
/
fk
. Therefore, the frequency of
fx
is given by
fx
=
fk
* (
N
/
J
).
S
o
u
rce
O
u
t
Co
u
nter 0
S
o
u
rce
G
a
te
O
u
t
Co
u
nter 1
S
ign
a
l to
Me
asu
re (
fx
)
S
ign
a
l of Known
Fre
qu
ency (
fk
)
CTR_0_
S
OURCE
(
S
ign
a
l to Me
asu
re)
CTR_0_OUT
(CTR_1_GATE)
CTR_1_
S
OURCE
Interv
a
l
to Me
asu
re
0
1
2
3
…
N
