ETAS
Functional Description
ES441.1 - User Guide
55
The following is true, with the frequency f and n periods per cycle:
x = f / n
The theoretical error e(x) is thus:
e(x) = x · 20 ns + 0.02 Hz / x
In the case of low frequencies, measurement accuracy is mainly governed by
the error of the integer division. In the case of high frequencies, the resolution
of the internal timebase of the ES441.1 has a greater influence on measure
-
ment accuracy.
Increasing Measurement Accuracy with Low Frequencies
When determining low frequencies, the result can be inaccurate due to the inte
-
ger calculations. This error can be avoided by measuring the period duration
instead of the frequency and calculation of the reciprocal value in the calibra
-
tion software on the PC (either as a formula in the A2L file or as a calculated
signal).
• Advantage of the procedure:
– No additional error by integer division
• Disadvantages of the procedure:
– A possible division by zero in the calibration software must be con
-
sidered,
– The Timeout function is not available for the frequency.
Increasing Measurement Accuracy with High Frequencies
When determining high frequencies, the result can be inaccurate due to mea
-
surement errors in the determination of the period duration by the internal time
-
base. This error can be avoided or diminished if the frequency is measured over
one cycle instead of over the period duration. The share of the measurement
error that is contributed by the internal timebase is then distributed to the n
periods of the cycle.
When measuring a cycle of frequency f
c
with n periods instead of a period of
the frequency f
p
, the following is true for the measurement result:
f
p
= n · f
c
NOTE
The measurement error can be very high when measuring very high frequen
-
cies or very short periods as the period duration of the signal to be measured
and the resolution of the internal timebase of the ES441.1 are of comparable
dimension. This is why, with high frequencies, the frequency should be deter
-
mined using the cycle duration and not the period duration.
