© National Instruments
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3-11
Settings
The settings available for a Semi-Period measurement are similar to those available for pulse
measurements. Refer to the
section for more information. For example, use
CI.SemiPeriod.Term to change the signal-to-measure.
Pulse measurements support sample clock timing; Semi-Period measurements do not.
Frequency Measurement
Frequency Measurement Considerations
The NI 6614 supports five methods for measuring frequency. Table 3-1 summarizes the five
frequency measurement methods.
In choosing a method, consider the measurement duration, timing, and number of counters.
Measurement duration can be either a fixed time or a fixed number of periods of the input signal.
That is, to calculate frequency, the counter can either:
•
measure the number of periods (
p
) that occur during a specified time duration (
t
).
Figure 3-13 shows an example with a measurement duration of 100 µs.
•
measure the time
(t
) it takes to observe a specified number of periods (
p
shows an example with a measurement duration of three periods.
Table 3-1.
Frequency Measurement Methods
Method
Timing
Number
of
Counters
Measurement
Duration
Sample
Clock
1
Time between
two sample clock pulses
Sample Clock (without Averaging)
1
1 period of input signal
Sample
Clock or
Implicit
1
1 period of input signal
Implicit
2
P periods of input signal
High Frequency with 2 Counters
2
T time