Chapter 7 Counters
NI 6238/6239 User Manual 7-12 ni.com

Figure 7-12. Method 1b

Method 2—Measure High Frequency with Two Counters

In this method, you measure one pulse of a known width using your signal

and derive the frequency of your signal from the result. This method is good

for high frequency signals.

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 or RTSI 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 pulseT to be N periods of F1. Then the frequency of F1 is N/T.

Figure 7-13 illustrates this method. Another option would be to measure

the width of a known period instead of a known pulse.

F1
Ft
F1
Intervals Measured
Gate
Source
Buffered Period
Measurement
Average Period of F1 =
N
1 +
N
2 + …
N
K
K
Frequency of F1 =
N
1 +
N
2 + …
N
K
K × Ft
Ft
T1T2…T
K
1
Ft
×
1 2 ...
N
11... ...
N
2 … 1... ...
N
K