Resolution Of Frequency Measurements; Resolution Of Duty Cycle Measurements - Campbell SDM-IO16 User Manual

SDM-IO16 16 Channel Input/Output Expansion Module
B.3 Resolution of Frequency Measurements
B.4 Resolution of Duty Cycle Measurements
B-2
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 s). The resultant resolution can be calculated
with the following equation:
2
f = f / (4096 * Int (t * f) )
res
Where:
f = resolution in (Hz).
res
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.
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
accurate duty cycle readings for higher frequency signals. Signals up to 4.000 kHz
can be measured without error. Signals of higher frequency will also appear to
give accurate measurements. Care should be taken as signals that are exact
multiples of the sampler frequency, e.g. 4096, 8192 Hz will give completely
spurious readings.