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Chapter 5
Counters
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.
Table 5-5 summarizes some of the differences in methods of measuring frequency.
For information about connecting counter signals, refer to the
section.
Period Measurement
In period measurements, the counter measures a period on its Gate input signal after the counter
is armed. You can configure the counter to measure the period between two rising edges or two
falling edges of the Gate input signal.
Table 5-5.
Frequency Measurement Method Comparison
Method
Number
of
Counters
Used
Number of
Measurements
Returned
Measures
High
Frequency
Signals
Accurately
Measures Low
Frequency
Signals
Accurately
Low frequency
with one counter
1
1
Poor
Good
High frequency
with two
counters
1 or 2
1
Good
Poor
Large range of
frequencies with
two counters
2
1
Good
Good
Sample clocked
(averaged)
1
1
Good
Good
