National Instruments cDAQ-9172 User Manual download pdf (Page 51)

Method 2—Measure High Frequency With Two Counters

This method is good for high frequency signals. Use this method to

measure one pulse of a known width using your signal and derive the

frequency of your signal from the result. Figure 26 illustrates this method.

Figure 26. Frequency Measurement—Method 2

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 (F1) to the Source of the counter. Configure the

counter for a single pulse-width measurement. Suppose you measure the

width of pulse T to be N periods of F1. Then the frequency of F1 is N/T.

Another option would be to measure the width of a known period instead

of a known pulse.

Method 3—Measure Large Range of Frequencies Using

Two Counters

By using two counters, you can accurately measure a signal that might

be high or low frequency. This technique is called reciprocal frequency

measurement. In this method, you generate a long pulse using the signal to

measure. You then measure the long pulse with a known timebase. The

NI cDAQ-9172 chassis can measure this long pulse more accurately than

the faster input signal.

Pulse

Gate

Source

1 2 … N

Pulse-Width

Measurement

T =

Frequency of F1 =

Width of

Pulse

Width of Pulse (T)

1 2 ... 46 47 48 49 50 51 52 53 54 55 56 ... 83 84

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National Instruments cDAQ-9172 User Manual Page 51