© National Instruments
|
7-11
M Series User Manual
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 measure, the desired accuracy, how many counters are available, and
how long the measurement can take. For all frequency measurement methods, assume the
following:
fx
is the frequency to be measured if no error
fk
is the known source or gate frequency
Measurement time (
T
)
is the time it takes to measure a single sample
Divide down (
N
)
is the integer to divide down measured frequency, only used in
large range two counters
Here is how these variables apply to each method, summarized in Table 7-1.
•
One counter
—With one counter measurements, a known timebase is used for the source
frequency (
fk
). The measurement time is the period of the frequency to be measured, or
1/
fx
.
•
One counter averaged
—Measuring
N
periods of F1 improves the accuracy of the
measurement.
•
Two counters high frequency
—With the two counter high frequency method, the second
counter provides a known measurement time. The gate frequency equals 1/
measurement
time
.
•
Two counters large range
—The two counter larger range measurement is the same as a
one counter measurement, but now the user has an integer divide down of the signal. An
internal timebase is still used for the source frequency (
fk
), but the divide down means that
the measurement time is the period of the divided down signal, or
N
/
fx
where
N
is the divide
down.
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