Chapter 8
Counters
©
National Instruments Corporation
8-11
•
Method 1b: Measure Low Frequency with One Counter
(Averaged)
—In this method, yo
u
meas
u
re several periods of yo
u
r
signal
u
sing a known timebase. This method is good for low to medi
u
m
freq
u
ency signals.
Yo
u
can ro
u
te the signal to meas
u
re (F1) to the Gate of a co
u
nter. Yo
u
can ro
u
te a known timebase (Ft) to the So
u
rce of the co
u
nter. The
known timebase can be 80MHzTimebase. For signals that might be
slower than 0.02 Hz,
u
se a slower known timebase.
Yo
u
can config
u
re the co
u
nter to make K + 1 b
u
ffered period
meas
u
rements. Recall that the first period meas
u
rement in the b
u
ffer
sho
u
ld be discarded.
Average the remaining K period meas
u
rements to determine the
average period of F1. The freq
u
ency of F1 is the inverse of the average
period. Fig
u
re 8-11 ill
u
strates this method.
Figure 8-11.
Method 1b
F1
Ft
F1
Interv
a
l
s
Me
asu
red
G
a
te
S
o
u
rce
B
u
ffered Period
Me
asu
rement
Aver
a
ge Period of F1 =
N
1
+
N
2
+ …
N
K
K
Freq
u
ency of F1 =
N
1
+
N
2
+ …
N
K
K
×
Ft
Ft
T
1
T
2
… T
K
1
Ft
×
1
2
...
N
1
1...
...
N
2
… 1...
...
N
K
