Divide down (N) is the integer to divide down measured frequency, only used in large
range two counters
fs is the sample clock rate, only used in sample clocked frequency
measurements
Here is how these variables apply to each method, summarized in the table below.
• 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 21. Frequency Measurement Methods
Variable Sample Clocked One Counter Two Counters
High
Frequency
Large Range
fk Known timebase Known
timebase
1
gating period
Known timebase
Measurement
time
1
fs
1
fx
gating period
N
fx
Max.
frequency
error
fx ×
fx
fk ×
fx
fs
− 1
fx ×
fx
fk − fx
fk
fx ×
fx
N × fk − fx
Max. error %
fx
fk ×
fx
fs
− 1
fx
fk − fx
fk
fx
fx
N × fk − fx
Note Accuracy equations do not take clock stability into account. Refer to the
specifications document for your chassis for information about clock stability.
cRIO-904x User Manual | © National Instruments | 89
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