National Instruments cRIO-904 Series User Manual

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.
Table 24. 5 MHz Frequency Measurement Methods
Method Comparison Sample Clocked
(Averaged)
One Counter Two Counters
High Frequency Large
Range
Number of counters
used
1 1 1 or 2 2
Number of
measurements returned
1 1 1 1
Measures high
frequency signals
accurately
Good Poor Good Good
Measures low frequency
signals accurately
Good Good Good Poor
cRIO-904x User Manual | © National Instruments | 91

National Instruments cRIO-904 Series Specifications

General

Brand	National Instruments
Model	cRIO-904 Series
Category	Controller
Language	English