National Instruments USB-621 Series - Page 109

Chapter 8 Counters
© National Instruments Corporation 8-11 NI USB-621x 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 8-11 illustrates this method.
Figure 8-11. Method 1b
F1
Ft
F1
Intervals Measured
Gate
Source
Buffered Period
Measurement
Average Period of F1 =
N
1
+ N
2
+ …N
K
K
Frequency 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