National Instruments Data Acquisition Device NI USB-621x, Руководство пользователя

Страница: 104 / 185

Chapter 8 Counters
© National Instruments Corporation 8-11 NI USB-621x User Manual
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
• Method 2: Measure High Frequency with Two Counters —In this
method, yo u meas u re one p u lse of a known width u sing yo u r signal
and derive the freq u ency of yo u r signal from the res u lt. This method is
good for high freq u ency signals.
In this method, yo u ro u te a p u lse of known d u ration (T) to the Gate of
a co u nter. Yo u can generate the p u lse u sing a second co u nter. Yo u also
can generate the p u lse externally and connect it to a PFI terminal. Yo u
only need to u se one co u nter if yo u generate the p u lse externally.
Ro u te the signal to meas u re (F1) to the So u rce of the co u nter.
Config u re the co u nter for a single p u lse-width meas u rement. S u ppose
yo u meas u re the width of p u lse T to be N periods of F1. Then the
freq u ency of F1 is N /T.
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