NI cDAQ-9178 - Page 44

NI cDAQ-9178/9174 User Guide and Specifications 44 ni.com
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 Table 5.
• 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).
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 the sample clocked (with averaging) and two counter frequency measurements are configured
the same, Table 6 summarizes the results.
Table 5. 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 information about
clock stability.
1
ating period
-----------------------------------
1
fs
------
1
fx
------
N
fx
------
fx
fx
fk
fx
fs
------ 1–×
----------------------------------
×
fx
fx
fk fx
–
------------------
×
fx
fx
Nfkfx
–×
----------------------------
×
fx
fk
fx
fs
------ 1–×
----------------------------------
fx
fk fx
–
------------------
fk
fx
------
fx
Nfkfx
–×
----------------------------