(decimal counting units). See figure 2. The time base frequency controls
the amount of time that the gate will remain open.
The gate, when it is
open, will allow events to pass from the input to the DCUs. The DCUs will
count the events and display them as a digital readout.
Figure 2.
For example, in a frequency measurement with an external input signal
of 20 kHz, and a time base setting of 1 second, the gate will open and allow
the events to be passed to the DCUs for 1 second. The DCUs will count and
display 20,000 events 1 count. For high frequencies, this method is very
accurate.
However, if the gate was open for 1 second, allowing a 1 Hz
external signal to pass to the DCUs, then the DCUs would display a reading
of one, with a possible error of 1 count. The reason for an error of 1
count is that it is possible for the gate to open and close at the same time
the pulses are to be counted, or the decay time of the gate closing may be
too long (see figure 3).
With this accuracy the counter may read 0 Hz (100 percent possible
error), 1 Hz or 2 Hz.
Therefore, frequency measurement is not a very
accurate means of measuring events of a low frequency signal.
b. Period Measurement.
Period measurement is more accurate than
frequency measurement for frequencies below 1 kHz due to the count error.
In period measurement, the counter counts the number of precise time base
events, with an accuracy of 1 microsecond, which occurs during one cycle
of the input frequency (see figure 4).
4
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