National Instruments USB-621 Series User Manual page 109

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• 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 8-11 illustrates this method.
F1
Gate
Ft
Source
Buffered Period
Measurement
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