Programming instructions

© National Instruments Corporation 26-1 LabVIEW Data Acquisition Basics Manual
26
Measuring Frequency
and Period
This chapter describes the various ways you can measure frequencies and
periods of TTL signals using the counters on your data acquisition (DAQ)
device. One cycle of a signal, known as the period, is measured in units of
time
— usually seconds. The inverse of period is frequency, which is
measured in cycles per second or hertz (Hz). The rate of your signal and the
type of counter on your DAQ device determine whether you use frequency
or period measurement. An example of when you would want to know the
frequency of a signal is if you need to monitor the shaft speed of the motor.
Knowing How and When to Measure
Frequency and Period
A common way to measure the frequency of a signal is to measure the
number of pulses that occur during a known time period. For example,
Figure 26-1 illustrates the measurement of a pulse train of an unknown
frequency (
f
s
) by using a pulse of a known width (
T
G
). The frequency of
the waveform equals the count divided by the known pulse width
(frequency = count/
T
G
). The period is always the reciprocal of the
measured frequency (period = 1/
f
s
). You typically use frequency
measurement for high frequency signals where the signal to be measured
is approaching or faster than the chosen internal timebase.
Figure 26-1.
Measuring Square Wave Frequency
GATE
SOURCE (CLK)
OUT
Count Register
T
G
input of unknown
frequency, f
s
pulse of known width