Choosing A Method For Measuring Frequency - National Instruments NI USB-621x User Manual

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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.
From Counter 0, the length of the pulse is N/F1. From Counter 1, the length
of the same pulse is J/F2. Therefore, the frequency of F1 is given by
F1 = F2 * (N/J).

Choosing a Method for Measuring Frequency

The best method to measure frequency depends on several factors including
the expected frequency of the signal to measures, the desired accuracy, how
many counters are available, and how long the measurement can take.
• Method 1 uses only one counter. It is a good method for many
applications. However, the accuracy of the measurement decreases as
the frequency increases.
Consider a frequency measurement on a 50 kHz signal using an
80 MHz Timebase. This frequency corresponds to 1600 cycles of the
SOURCE
COUNTER 0
SOURCE
GATE
0 1 2 3 ... N
Interval
to Measure
Figure 9-14. Method 3
9-13
Chapter 9
OUT
OUT
COUNTER 1
NI USB-621x User Manual
Counters