National Instruments USB-621 Series User Manual page 111

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• 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.
Signal to
Measure (F1)
Signal of Known
Frequency (F2)
CTR_0_SOURCE
(Signal to Measure)
CTR_0_OUT
(CTR_1_GATE)
CTR_1_SOURCE
Then route the Counter 0 Internal Output signal to the Gate input of
Counter 1. You can route a signal of known frequency (F2) to the
Counter 1 Source input. F2 can be 80MHzTimebase. For signals that
might be slower than 0.02 Hz, use a slower known timebase. Configure
Counter 1 to perform a single pulse-width measurement. Suppose the
result is that the pulse width is J periods of the F2 clock.
SOURCE
COUNTER 0
SOURCE
GATE
0 1 2 3 ... N
Interval to
Measure
Figure 8-13. Method 3
8-13
Chapter 8
OUT
OUT
COUNTER 1
NI USB-621x User Manual
Counters