Campbell Scientific SDM-IO16 16-Channel Input/Output Expansion Module - Manual (Page 36)

Appendix A. General Principles of Pulse and Frequency Measurements
A.3 Resolution of Frequency Measurements
The module measures frequency by counting the number of full signal cycles
between requests for measurements by the datalogger and measuring the time
between the start of the first and end of the last of these cycles. The resolution
of a frequency measurement will be dependent on the number of pulses and the
resolution of the internal timer (244
μs). The resultant resolution can be
calculated with the following equation:
f
res
= f 2 / (4096 * Int (t * f) )
Where:
f
res
= resolution in (Hz).
f = actual frequency measured in (Hz).
t = time between frequency measurement commands in (seconds).
Int = a function which returns the truncated integer value.
For example, reading 1000 Hz at a 0.25 sec frequency measurement interval
will give a resolution of 0.97 Hz, whilst at 1 sec between measurements the
resolution would be 0.25 Hz.
The resolution improves with longer times between frequency measurement
commands. However, the maximum time between measurements is 15.9375
seconds which is limited by the range of internal counters.
A.4 Resolution of Duty Cycle Measurements
Duty cycle measurements are made by calculating the proportion of time that a
signal is high for all full signal cycles that occur in between two measurement
requests by the datalogger. The resolution can be calculated using the
following equation:
%r = 100 * f / (4096*Int(t*f))
Where:
%r = duty cycle resolution in (%)
f = frequency of the signal in (Hz)
t = time between duty cycle measurement commands in (seconds)
Int = a function which returns the truncated integer value.
For example reading the duty cycle of a 1000 Hz signal at 0.25 sec intervals
will give a resolution of 0.097%, whilst at 1 second intervals the resolution
would be 0.025%.
It can be seen that duty cycle resolution improves with longer times between
duty cycle measurement commands. However, the maximum time between
measurements is 15.9375 seconds which is limited by the range of internal
counters.
Although the duty cycle measurement uses the same sampling frequency as the
frequency measurement technique, it is not dependent on counting or timing a
known number of cycles. For this reason, it is capable of sampling and giving
A-2