National Instruments 6612, User Manual

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NI 6612 User Manual
Frequency Measurement
Frequency Measurement Considerations
The NI 6612 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 ). Figure 3-14
shows an example with a measurement duration of three periods.
In both cases, the frequency, f , is given by:
Figure 3-13. Frequency Measurement Using Time
Table 3-1. Frequency Measurement Methods
Method Timing
Number
of
Counters
Measurement
Duration
Sample Clock (with Averaging) Sample
Clock 1 Time between
two sample clock pulses
Sample Clock (without Averaging) 1 1 period of input signal
Low Frequency with 1 Counter Sample
Clock or
Implicit 1 1 period of input signal
Large Range with 2 Counters Implicit 2 P periods of input signal
High Frequency with 2 Counters 2 T time
f
p
t
--- =
Co u nt
S ign a l to Me asu re
Co u nter Time bas e
(100 MHz)
me asu rement d u r a tion = 100 µ s
1
0
2
50