WHITE BROWN DA 918 Manual do Utilizador descarregar pdf (Página 80)

NI cDAQ-918x/919x User Manual

• Large Range of Frequencies with Two Counters

• Sample Clocked Buffered Frequency Measurement

Low Frequency with One Counter

For low frequency measurements with one counter, you measure one period of your signal using

a known timebase.

You can route the signal to measure (fx) to the Gate of a counter. You can route a known timebase

(fk) to the Source of the counter. The known timebase can be an onboard timebase, such as

80 MHz Timebase, 20 MHz Timebase, or 100 kHz Timebase, or any other signal with a known

rate.

You can configure the counter to measure one period of the gate signal. The frequency of fx is

the inverse of the period. Figure 5-12 illustrates this method.

Figure 5-12. Low Frequency with One Counter

High Frequency with Two Counters

For high frequency measurements with two counters, you measure one pulse of a known width

using your signal and derive the frequency of your signal from the result.

Note Counter 0 is always paired with Counter 1. Counter 2 is always paired with

Counter 3.

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 generate the pulse externally and connect

it to a PFI terminal. You only need to use one counter if you generate the pulse externally.

Route the signal to measure (fx) to the Source of the counter. Configure the counter for a single

pulse-width measurement. If you measure the width of pulse T to be N periods of fx, the

frequency of fx is N/T.

Gate

Source

123 … N

Single Period

Measurement

…

Period of fx =

Frequency of fx =

Interval Measured

1 2 ... 75 76 77 78 79 80 81 82 83 84 85 ... 126 127

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