National Instruments NI cRIO-905x Series User Manual page 81

large range are almost the same. The advantage of the sample clocked method is that even
when the frequency to measure changes, the measurement time does not and error percentage
varies little. For example, if you configured a large range two-counter measurement to use a
divide down of 50 for a 50 k signal, then you would get the accuracy measurement time and
accuracy listed in the 50 kHz Frequency Measurement Methods table. But if your signal
ramped up to 5 M, then with a divide down of 50, your measurement time is 0.01 ms, but your
error is now 0.125%. The error with a sample clocked frequency measurement is not as
dependent on the measured frequency so at 50 k and 5 M with a measurement time of 1 ms the
error percentage is still close to 0.00125%. One of the disadvantages of a sample clocked
frequency measurement is that the frequency to be measured must be at least twice the sample
clock rate to ensure that a full period of the frequency to be measured occurs between sample
clocks.
• Low frequency measurements with one counter is a good method for many applications.
However, the accuracy of the measurement decreases as the frequency increases.
• High frequency measurements with two counters is accurate for high frequency signals.
However, the accuracy decreases as the frequency of the signal to measure decreases. At
very low frequencies, this method may be too inaccurate for your application. Another
disadvantage of this method is that it requires two counters (if you cannot provide an
external signal of known width). An advantage of high frequency measurements with two
counters is that the measurement completes in a known amount of time.
• Measuring a large range of frequencies with two counters measures high and low
frequency signals accurately. However, it requires two counters, and it has a variable
sample time and variable error % dependent on the input signal.
The following table summarizes some of the differences in methods of measuring frequency.
Method Comparison
Number of counters
used
Number of
measurements returned
Measures high
frequency signals
accurately
Measures low frequency
signals accurately
Table 21. 5 MHz Frequency Measurement Methods
Sample Clocked
(Averaged)
1
1
Good
Good
One Counter
High Frequency
1
1
Poor
Good
NI cRIO-905x User Manual | © National Instruments | 81
Two Counters
Large
Range
1 or 2 2
1 1
Good Good
Good Poor