General Principles Of Pulse And Frequency; Measurements; Introduction; Frequency And Duty Cycle Measurement Range - Campbell SDM-IO16 User Manual

Appendix B. General Principles of Pulse
and Frequency Measurements
B.1 Introduction
B.2 Frequency and Duty Cycle Measurement Range
It is necessary to understand the general method of input measurements of the
SDM-IO16 to be able to easily comprehend the limits of frequency and duty cycle
resolution.
The microprocessor in the module runs an internal task that reads the status of all
16 ports at a fixed frequency of 4096 Hz. Changes of state of each port from one
sample to the next are used to determine the start and end of pulses. This sampling
frequency determines the resolution and range of the pulse measurements.
To guarantee that a pulse is detected it must last longer than the time between
samples which is 244 s. This sets the upper limit of signal frequency for which
pulses can be counted or frequencies measured. By implication, the maximum
frequency that can be measured is with a 50/50 duty cycle signal. If the duty cycle
is different from this, the maximum frequency measurable is lower. This
maximum frequency, measurable for a signal with a range of duty cycles, can be
expressed as the minimum of two functions:
f = %min * 4096 / 100 (1)
max
f = (100 - %max) * 4096 / 100
max
Where:
f = maximum frequency at a specific duty cycle (Hz)
max
%min = minimum duty cycle in %
%max = maximum duty cycle in %
It also follows that for any given frequency (f) there will be a limit to the
maximum and minimum duty cycle that can be measured due to the restriction of
the minimum detectable pulse width. Using the same variables defined above,
%min = f * 100 / 4096
%max = 100 - %min
It can be seen that the lower the frequency, the larger the measurable range of duty
cycle.
(2)
B-1