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Chapter 26
Measuring Frequency and Period
TIO-ASIC, DAQ-STC, Am9513
For period measurement, you count the number of pulses of a known
frequency (fs) 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 (TG = count/fs).
GATE
OUT
TG
Count Register
SOURCE
input of known
frequency, fs
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 TIO-ASIC are 20 MHz, 100 kHz,
and a device-specific maximum 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.
1
period measurement = -----------------------------------------------------------frequency measurement
1
frequency measurement = --------------------------------------------------period measurement
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.
LabVIEW Data Acquisition Basics Manual
26-2
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