High Frequency With Two Counters; Large Range Of Frequencies With Two Counters - National Instruments NI cDAQ-9181 User Manual

Chapter 5 Counters

High Frequency with Two Counters

For high frequency measurements with two counters, you measure one pulse of a known width
using your signal and derive the frequency of your signal from the result.
Counter 0 is always paired with Counter 1. Counter 2 is always paired with
Note
Counter 3.
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 (fx) to the Source of the counter. Configure the counter for a single
pulse-width measurement. If you measure the width of pulse T to be N periods of fx, the
frequency of fx is N/T.
Figure 5-13 illustrates this method. Another option is to measure the width of a known period
instead of a known pulse.
Pulse
Gate
fx
Source
Pulse-Width
Measurement

Large Range of Frequencies with 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. When measuring a large range of
frequencies with two counters, you generate a long pulse using the signal to measure. You then
measure the long pulse with a known timebase. The cDAQ chassis can measure this long pulse
more accurately than the faster input signal.
Counter 0 is always paired with Counter 1. Counter 2 is always paired with
Note
Counter 3.
5-12 | ni.com
Figure 5-13. High Frequency with Two Counters
Pulse
fx
Width of Pulse (T )
1 2 ...
Width of
T =
Pulse
Frequency of fx =
N
N
fx
N
T