Specifications
8-162 | OWNER’S GUIDE
Frequency Measurement
In general it is possible to take the inverse of a period measurement to get the frequency of the input signal.
If the period time measurement is done an inaccuracy of counted internal time base cycles of ±1 cycle ap-
pears, because the counted cycles of the internal time base depends on the phase of the input signal with
re- spect to the internal time base. For long period times, and therewith low frequencies, the measurement
error is negligible. At high frequencies, and therewith short period times, few cycles are counted. In this case
the error of ±1 cycle becomes signicant.
Accuracy at period time measurement gure:
For higher precision result the frequency measurement is done with two counters. In each case two coun-
ters are paired, i.e. it have to be used counter 0 and counter 1 or counter 2 and counter 3 or counter 4 and
counter 5 or counter 6 and 7 for the frequency measurement. The rst coun- ter counts the rising edges on
Counter Source. The second counter counts the rising edges of the internal time base. At every rising edge
on Counter Source the counter value of the second counter is stored in a register. At every Sample Clock
((1), (2), ... (6)) the values of both counters are read out.
Frequency measurement gure:
With these both measurement results not only the frequency can be calculated in a precise way. Also the
event counter result can be show in fractions because the exact time when the event occurs at the input is
known. The event counting result is recalculated with interpolation to the exact sample point like shown in
the diagram above.