National Instruments Data Acquisition Device NI USB-621x User Manual page 104

© National Instruments Corporation
Average the remaining K period measurements to determine the
average period of F1. The frequency of F1 is the inverse of the average
period. Figure 8-11 illustrates this method.
F1
Gate
Ft
Source
Buffered Period
Measurement
• Method 2: Measure High Frequency with Two Counters—In this
method, you measure one pulse of a known width using your signal
and derive the frequency of your signal from the result. This method is
good for high frequency signals.
In this method, you route a pulse of known duration (T) to the Gate of
a counter. You can generate the pulse using a second counter. You also
can generate the pulse externally and connect it to a PFI terminal. You
only need to use one counter if you generate the pulse externally.
Route the signal to measure (F1) to the Source of the counter.
Configure the counter for a single pulse-width measurement. Suppose
you measure the width of pulse T to be N periods of F1. Then the
frequency of F1 is N/T.
Intervals Measured
T
1
F1
1 2 ...N 1... ...N
1
Ft
Average Period of F1 =
Frequency of F1 =
Figure 8-11. Method 1b
8-11
Chapter 8
T ... T
2 K
... 1... ...N
2 K
N + N + ...N
1 2 K
×
K
K × Ft
N + N + ...N
1 2 K
NI USB-621x User Manual
Counters
1
Ft