72 CBLé System Guidebook
Miscellaneous Reference Information
(Continued)
Measuring Period and Frequency
Period and frequency apply only to
CH1
and only
CH1
can be active if
Operation is set to 5 (Period) or 6 (Frequency). Period and frequency are
measured on Vin pin (pin 1) of
CH1
. Period and frequency measurements
always use the hardware threshold.
The CBL measures period and frequency by counting edges for 0.25
seconds, or measuring the time between the selected edges for one
period—whichever is larger (see figure below). If a significant number of
edges are counted during the 0.25-second period, the count is used to
compute both period and frequency; otherwise, the period and frequency
are computed from the time interval for one period.
0
1
2
3
T=
+
-
+
-
Trigger
Type
2
3
4
5
Measuring
Points
+ + (T=0 to 2)
N N (T=1 to 3)
+ N (T=0 to 1)
N + (T=1 to 2)
The crossover point between the two computations is about 600 Hz.
Because there can be a one-count uncertainty during the 0.25-second
period, the accuracy around 600 Hz is approximately ±4 Hz (about 0.7%).
The resolution of the timer measuring the time between edges is 6.4
microseconds; therefore, the percentage accuracy improves for
frequencies above and below 600 Hz.
If the CBL is set up using
CMD3
to make multiple measurements at a
particular sample time, the CBL waits for the sample time that you
specified after it completes the current measurement. It then initiates
the next cycle of period/frequency measurement. The minimum sampling
time for period and frequency is 0.25 seconds.
Note: Period and frequency measurements using Trigger Type 4 or 5 are
only possible on nonrepetitive signals or on repetitive signals that are
less then 600 Hz. This is because at 600 Hz, the edge counts will prevail.
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