Chapter 7
Counters
©
National Instruments
7-19
X Series User Manual
Here is how these variables apply to each method, s
u
mmarized in
Table 7-2.
•
One counter
—With one co
u
nter meas
u
rements, a known timebase is
u
sed for the so
u
rce freq
u
ency (
fk
). The meas
u
rement time is the period
of the freq
u
ency to be meas
u
red, or 1/
fx
.
•
Two counter high frequency
—With the two co
u
nter high freq
u
ency
method, the second co
u
nter provides a known meas
u
rement time. The
gate freq
u
ency eq
u
als 1/
measurement time
.
•
Two counter large range
—The two co
u
nter larger range
meas
u
rement is the same as a one co
u
nter meas
u
rement, b
u
t now the
u
ser has an integer divide down of the signal. An internal timebase is
still
u
sed for the so
u
rce freq
u
ency (
fk
), b
u
t the divide down means that
the meas
u
rement time is the period of the divided down signal, or
N
/
fx
where
N
is the divide down.
•
Sample clocked
—For sample clocked freq
u
ency meas
u
rements, a
known timebase is co
u
nted for the so
u
rce freq
u
ency (
fk
). The
meas
u
rement time is the period of the sample clock (
fs
).
Table 7-2.
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.
freq
u
ency
error
fk
Max. error %
Note
: Acc
u
racy eq
u
ations do not take clock stability into acco
u
nt. Refer to yo
u
r device specifications for clock stability.
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
–
×
-------------------------
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