Omega OMB-DAQBOARD-3000 Series User Manual page 57

PERIOD: OPT[1:0]: Determines the number of periods to time, per measurement. This makes it
possible to average out jitter in the input waveform, sampling error, noise, etc. There are four options:
(1) The channel's measurement is latched every time one complete period has been observed.
(2) The channel's measurement is latched every time that 10 complete periods have been observed.
The value that gets returned is equal to 10 consecutive periods of the input channel.
(3) The number returned is 100 consecutive periods.
(4) The number returned is 1000 consecutive periods.
PERIOD: OPT2: Determines whether the period is to be measured with a 16-bit (Counter Low), or
32-bit (Counter High) counter. Since period measurements always have the "stop at the top" option
enabled, this option dictates whether the measurement has a range of 0 to 65535 ticks or 0 to
4,294,967,295 ticks.
PERIOD: OPT4: Allows the mapped channel to gate the counter if desired. When the mapped
channel is high, the counter is enabled. When the mapped channel is low, the counter is disabled, but still
holds the count value. The mapped channel can be any other input channel.
PERIOD: OPT6: This allows a mapped channel's period to be measured, instead of the input channel.
The mapped channel can be any other input channel (post debounce). This option allows the counter to be
used with any other input channel (post-debounce). If the channel's input is used elsewhere, for example,
gating another counter, the counter for this channel does not need to go unused.
Period and Frequency Accuracy
The /3000 Series board can measure the period of any input waveform. It does this by counting the
integral number of "ticks" that make up the period, the data returned will always be time measured in
"ticks." The error in each data sample will come from two sources: the sampling error caused by not
being able to count a partial "tick"; and the 3000 Series Board's internal timebase inaccuracy. The board's
internal timebase has an absolute accuracy of 30 ppm. The sampling error will vary with input frequency,
selected ticksize, and selected averaging mode. The absolute error is the "root-sum-of-squares" of the two
independent error sources. For example, if the sampling error is 30 ppm and the timebase accuracy is 30
ppm, the absolute accuracy is 42 ppm.
Many times the desired accuracy is much less than what the internal timebase is capable of. Other
applications will require a more accurate period measurement and the effects of sampling error will have to
be averaged out leaving only the inaccuracy associated with the internal timebase. Inaccuracy due to the
internal timebase cannot be averaged out.
For period and frequency measurements, percent sampling error is equal to 100%/(n+1) where n=0 to
65,535 for a 16-bit counter and n=0 to 4,294,967,295 for a 32-bit counter. For small count values, the
sampling error is large and for large count values, the sampling error is small. If sampling error is to be
less than 0.21%, n must be greater than 480 regardless of counter size.
Sampling error can also be reduced by averaging many samples together. Assuming the input signal is
asynchronous to the board's internal timebase, sampling error can be divided by the square-root of the
number of samples taken. The averaging can be done with PC-based software.
The board has the ability to measure 1, 10, 100 or 1000 periods, dividing the sampling error by 1, 10, 100,
or 1000. This is done within the board circuitry and may eliminate the need for any averaging to be done
in the PC. For high accuracy on high frequency inputs, multiple period measurement and PC-based
averaging can be done.
The 3000 Series board has the ability to provide various frequency ranges that are based upon different
ticksizes, averaging options, and counter size (16 bit or 32 bit values.) The frequency ranges are designed
to fit a wide array of possible applications. Within each range, the sampling error decreases dramatically
as the input period increases. The ranges will get smaller as required accuracy increases.
DaqBoard/3000 Series User's Manual
918494
Counter Input Modes 5-9