National Instruments NI USB-621x manual 92

Chapter 9 Counters

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.

Figure 9-13illustrates this method. Another option would be to measure the width of a known period instead of a known pulse.

					Width of Pulse (T)
Pulse		Pulse
	Gate
		1	2			…		N
F1	Source	F1
			Width of T =				N
	Pulse-Width
	Measurement			Pulse			F1

Frequency of F1 =

N

T

Figure 9-13. Method 2

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

MSeries device can measure this long pulse more accurately than the faster input signal.

You can route the signal to measure to the Source input of Counter 0, as shown in Figure 9-14.Assume this signal to measure has frequency F1. Configure Counter 0 to generate a single pulse that is the width of N periods of the source input signal.

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