National Instruments NI USB-621x - Manual (Page 92)

Chapter 9 Counters
NI USB-621x User Manual 9-12 ni.com
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 9-13 illustrates this method. Another option would be to measure
the width of a known period instead of a known pulse.
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
M Series 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.
Pulse
F1
Pulse
F1
Gate
Source
1 2 … N
Pulse-Width
Measurement
T =
N
F1
Frequency of F1 =
T
Width of
Pulse
N
Width of Pulse (T)