Programming instructions

Chapter 26 Measuring Frequency and Period
LabVIEW Data Acquisition Basics Manual 26-2
© National Instruments Corporation
DAQ-STC, Am9513
For period measurement, you count the number of pulses of a known
frequency (
f
s
) during one period of the signal to be measured. As shown in
Figure 26-2, the signal of a known frequency is connected to the SOURCE,
and the signal to be measured is connected to the GATE. The period is the
count divided by the known frequency (
T
G
= count/
f
s
).
Figure 26-2.
Measuring a Square Wave Period
You typically use period measurement for low frequency signals where
the signal to be measured is significantly slower than the chosen internal
timebase. The internal timebases for the DAQ-STC are 20 MHz and
100 kHz. The internal timebases for the Am9513 are 1 MHz, 100 kHz,
10 kHz, 1 kHz, and 100 Hz. Whether you use period measurement or
frequency measurement, you can always obtain the other measurement by
taking the inverse of the current one as shown in the following equations.
8253/54
The 8253/54 chip does not support period measurement, but you can use
frequency measurement for a pulse train and take the inverse to get the
period. The frequency examples discussed in this chapter calculate the
period for you.
GATE
SOURCE
OUT
Count Register
T
G
input of known
frequency, f
s
period measurement
1
frequency measurement
------------------------------------------------------------=
frequency measurement
1
period measurement
---------------------------------------------------=