National Instruments PCI-6238, Руководство пользователя

Страница: 77 / 173

Chapter 7 Counters
© National Instruments Corporation 7-11 NI 6238/6239 User Manual
You can route the signal to measure (F1) to the Gate of a counter. You can
route a known timebase (Ft) to the Source of the counter. The known
timebase can be 80MHzTimebase. For signals that might be slower than
0.02 Hz, use a slower known timebase.
You can configure the counter to measure one period of the gate signal. The
frequency of F1 is the inverse of the period. Figure 7-11 illustrates this
method.
Figure 7-11. Method 1
Method 1b—Measure Low Frequency with One
Counter (Averaged)
In this method, you measure several periods of your signal using a known
timebase. This method is good for low to medium frequency signals.
You can route the signal to measure (F1) to the Gate of a counter. You can
route a known timebase (Ft) to the Source of the counter. The known
timebase can be 80MHzTimebase. For signals that might be slower than
0.02 Hz, use a slower known timebase.
You can configure the counter to make K + 1 buffered period
measurements. Recall that the first period measurement in the buffer should
be discarded.
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 7-12 illustrates this method.
F1
Ft
F1
Ft
Gate
Source
1 2 3 … N
Single Period
Measurement
…
Period of F1 =
N
Ft
Frequency of F1 =
N
Ft
Interval Measured