National Instruments PXI-6289 User Manual page 109

Chapter 7 Counters
Variable
fk
T
Maximum
fx
frequency
error (Hz)
Maximum
error (%)
Note: Accuracy equations do not take clock stability into account. Refer to your device specifications for
clock stability.
Which Method Is Best?
This depends on the frequency to be measured, the rate at which you want to monitor the
frequency and the accuracy you desire. Take for example, measuring a 50 kHz signal. Assuming
that the measurement times for all measurements are configured the same, Table 7-2
summarizes the results.
Table 7-2. 50 kHz Frequency Measurement Methods
Variable
fx
fk
Measurement
time (mS)
N
7-12 | ni.com
Table 7-1. Frequency Measurement Methods
One Counter
—
Known Known timebase
timebase
1
--- -
fx
fx
fx
× -------------- -
fk fx
–
fx
-------------- -
fk fx
–
One Counter
—
50,000
100 M
0.02
—
Averaged Frequency
-------------------------------
gating period
N
+ 1
------------ -
gating period
fx
fx
------------------------ -
×
N fk fx
× –
fx
------------------------ -
N fk fx
× –
Averaged
50,000
100 M
1.02
50
Two Counters
High
Large Range
1 Known timebase
fk fx
×
fk
--- - ------------------------ -
fx N fk fx
Two Counters
High
Frequency Large Range
50,000
1,000
1
—
N
--- -
fx
fx
------------------------ -
N fk fx
× –
fx
× –
50,000
100 M
1
50