Method 3-Measure Large Range Of Frequencies Using Two Counters - National Instruments NI 6221 User Manual

Chapter 7 Counters
M Series User Manual
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
M Series 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 7-13. 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 7-13. Method 3
7-12
OUT
OUT
COUNTER 1
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