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If a conversion equation is enabled for the trigger channel, then the threshold
specified in Command 3 should be a converted level. For example, if a pH probe is
plugged into CH 2 with a conversion equation loaded into CBL 2 and the trigger
channel specified as CH 2, the threshold level should be entered as a pH level in the
range 0-14, not as a voltage in the range 0-5V.
Measuring Period and Frequency
Period and frequency apply only to CH1 and only CH1 can be active if the 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 2 measures period and frequency by counting edges for 0.25 seconds, or by
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.
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 2 is set up using Command 3 to make multiple measurements at a particular
sample time, the CBL 2 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 non-
repetitive signals or on repetitive signals that are less then 600 Hz. This is because at 600 Hz, the
edge counts will prevail.
CBL 2 Technical Reference
+
-
T=
0
1
Figure 2. Period and Frequency Measurement
Trigger Type
2
3
4
5
+
-
3
2
Measuring Points
+ +
(T=0 to 2)
N N (T=1 to 3)
N + N (T=0 to 1)
N N + (T=1 to 2)
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