Programming instructions

Chapter 26 Measuring Frequency and Period
© National Instruments Corporation 26-9 LabVIEW Data Acquisition Basics Manual
or terminal count (TC). With a timebase of 20 MHz, the DAQ-STC
can measure a period up to 838 ms. With a timebase of 100 kHz,
you can measure a period up to 167 seconds.
Am9513
Figure 26-10 shows the example Measure Period-Easy (9513) VI
located in
labview\examples\daq\Am9513.llb
. This example uses
the Easy VI, Measure Pulse Width or Period located in
Functions»
Data Acquisition»Counter
.
Figure 26-10.
Diagram of Measure Period-Easy (9513) VI
You connect your signal of unknown period to the GATE of
counter
. The
counter measures the period between successive rising edges of your TTL
signal by counting the number of internal
timebase
cycles that occur during
the period. The
period
is the count divided by the timebase. The
frequency
is determined by taking the inverse of the
period
. The
valid?
output
indicates if the period was measured without overflow. Overflow occurs
when the counter reaches its highest value, or terminal count (TC). You
must choose
timebase
such that it does not reach TC. With a timebase of
1 MHz, the Am9513 can measure a period up to 65 ms. With a timebase of
100 Hz, you can measure a period up to 655 seconds.