National Instruments Eight-slot USB Chassis NI cDAQ-9172 - Manual (Page 51)

© National Instruments Corporation 51 NI cDAQ-9172 User Guide and Specifications
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
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)