National-instruments Data Acquisition Device NI USB-621x Instrukcja Użytkownika Pobierz pdf (Strona 102)

Chapter 8 Counters

Figure 8-9 shows an example of a buffered semi-period measurement.

Figure 8-9. Buffered Semi-Period Measurement

Note that if you are using an external signal as the Source, at least one

Source pulse should occur between each active edge of the Gate signal.

This condition ensures that correct values are returned by the counter. If this

condition is not met, the counter returns a zero. Refer to the Duplicate

Count Prevention section for more information.

For information about connecting counter signals, refer to the Default

Counter/Timer Pinouts section.

Frequency Measurement

You can use the counters to measure frequency in several different ways.

You can choose one of the following methods depending on your

application:

• Method 1: Measure Low Frequency with One Counter—In this

method, you measure one period of your signal using a known

timebase. This method is good for low 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.

SOURCE

GATE

Counter Value

Buffer

1 3

1 1

1 3

1 2

ounter Armed

1 2 ... 97 98 99 100 101 102 103 104 105 106 107 ... 184 185

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National-instruments Data Acquisition Device NI USB-621x Instrukcja Użytkownika Strona 102