Method 1B-Measure Low Frequency With One Counter (Averaged) - National Instruments NI 6221 User Manual

Chapter 7 Counters
M Series User Manual
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 7-11 illustrates this method.
F1
Gate
Ft
Source
Buffered Period
Measurement
Intervals Measured
T
1
F1
1 2 ... N
Ft
Average Period of F1 =
Frequency of F1 =
Figure 7-11. Method 1b
7-10
T ... T
2 K
1... ... N ... 1... ... N
1 2 K
N + N + ... N
1 2 K
K
K × Ft
N + N + ... N
1 2 K
1
×
Ft
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