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

Chapter 7 Counters

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
100 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. Figure 7-12 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 can also 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 (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.
7-12 | ni.com
Figure 7-12. Low Frequency with One Counter
fx
1
fk
Interval Measured
2 3 ...
N
Period of fx =
fk
Frequency of fx =
... N
fk
N