Low Frequency With One Counter; High Frequency With Two Counters - National Instruments cRIO-904 Series User Manual

Low Frequency with One Counter

For low frequency measurements with one counter, you measure one period of your signal
using a known timebase.
You can route the signal to measure (fx) to the Gate of a counter. You can route a known
timebase (fk) to the Source of the counter. The known timebase can be an onboard timebase,
such as 80 MHz Timebase, 20 MHz Timebase, or 100 kHz Timebase, or any other signal with
a known rate.
You can configure the counter to measure one period of the gate signal. The frequency of fx is
the inverse of the period. The following figure illustrates this method.
fx
Gate
fk
Source
Single Period
Measurement

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.
Note
Counter 0 is always paired with Counter 1. Counter 2 is always paired with
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.
The image below illustrates this method. Another option is to measure the width of a known
period instead of a known pulse.
Figure 60. Low Frequency with One Counter
fx
1
fk
Interval Measured
2 3 ...
N
Period of fx =
fk
fk
Frequency of fx =
N
cRIO-904x User Manual | © National Instruments | 85
... N