Causes of error.
As you can see, the blocks used for the period and frequency
measurements are the same major blocks. The only difference is which of the
two input signals is controlling the opening and closing of the gate.
In a period measurement, the time base must be variable in order to
increase the accuracy of the reading.
Here is an example of what would
happen if the time base was not variable. If a 1 Hz external signal is used
to open the gate and a 1 Hz time-base pulse is passed to the DCUs, the count
would be 1 event 1 count. Again, it may be 2 Hz, 1 Hz, or 0 Hz on the
display. However, if the time base was variable so that pulses of different
duration might be selected when the gate is open, then pulses with shorter
time duration can be selected. This way, more events would be passed to the
DCUs to be counted, resulting in a larger count and improving the accuracy.