National Instruments Data Acquisition Device NI USB-621x - Manual (Page 105)

Chapter 8 Counters
NI USB-621x User Manual 8-12 ni.com
Figure 8-12 illustrates this method. Another option would be to
measure the width of a known period instead of a known pulse.
Figure 8-12. Method 2
• Method 3: Measure Large Range of Frequencies Using Two
Counters—By using two counters, you can accurately measure a
signal that might be high or low frequency. This technique is called
reciprocal frequency measurement. In this method, you generate a long
pulse using the signal to measure. You then measure the long pulse
with a known timebase. The USB-621x device can measure this long
pulse more accurately than the faster input signal.
You can route the signal to measure to the Source input of Counter 0,
as shown in Figure 8-13. Assume this signal to measure has frequency
F1. Configure Counter 0 to generate a single pulse that is the width of
N periods of the source input signal.
Pulse
F1
Pulse
F1
Gate
Source
1 2 … N
Pulse-Width
Measurement
T =
N
F1
Frequency of F1 =
T
Width of
Pulse
N
Width of Pulse (T)