Chapter 7 Counters
© National Instruments 7-19 X Series User Manual
Here is how these variables apply to each method, summarized in
Table 7-2.
• One counter—With one counter measurements, a known timebase is
used for the source frequency (fk). The measurement time is the period
of the frequency to be measured, or 1/fx.
• Two counter high frequency—With the two counter high frequency
method, the second counter provides a known measurement time. The
gate frequency equals 1/measurement time.
• Two counter large range—The two counter larger range
measurement is the same as a one counter measurement, but now the
user has an integer divide down of the signal. An internal timebase is
still used for the source frequency (fk), but the divide down means that
the measurement time is the period of the divided down signal, or N/fx
where N is the divide down.
• Sample clocked—For sample clocked frequency measurements, a
known timebase is counted for the source frequency (fk). The
measurement time is the period of the sample clock (fs).
Table 7-2. Frequency Measurement Methods
Varia ble Sample Clocked One Counter
Two Counter
High
Frequency
Large Range
fk Known timebase Known timebase Known timebase
Measurement
time
gating period
Max.
frequency
error
fk
Max. error %
Note: Accuracy equations do not take clock stability into account. Refer to your device specifications for clock stability.
1
gating period
-------------------------------
fx
fx
fk
fx
fs
---- 1–×
-------------------------------
×
fx
fx
fk fx–
---------------
×
fx
fx
Nfkfx–×
-------------------------
×
fx
fk
fx
fs
---- 1–×
-------------------------------
fx
fk fx–
---------------
fx
Nfkfx–×
-------------------------
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