National Instruments NI USB-621x manual 91

Chapter 9 Counters

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 9-12illustrates this method.

T1

Intervals Measured

T2

…

TK

F1

Gate

F1

Ft

Source

12...N11......N2

… 1......NK

Ft

Buffered Period

Measurement

Average Period of F1 =

N1 + N2

+ …NK

⋅

1

K

Ft

K ⋅ Ft

Frequency of F1 =

N1 + N2 + …NK

Figure 9-12. Method 1b

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

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