Measurement Computing WBK17 - Manual (Page 16)

WBK17, pg. 16 987996 WBK17, Counter/Encoder Module
Each frequency range given in the previous table-set can be exceded. If the input waveform goes under-
range by too much, the counter value will top out at 65535 indicating you have reached the lowest possible
frequency that can be measured on that range. If the input waveform goes over range by too much, the
counter will return values that are very course and have a lot of sampling error. The values returned will
have a small number of counts for the period duration. If an input waveform cannot fit within one of the
16-bit ranges shown above or requires much higher accuracy, then a 32-bit range should be considered.
Full 32-bit Counter
Range (Hz)
Ticksize (nS) Averaging
Option
15u – 100 20000 1
150u – 1k 2000 1
1.5m – 10k 200 1
15m – 100k 20 1
150m – 1M 20 10
1.5 – 5M 20 100
15 – 5M 20 1000
Frequency Ranges for a 32-bit Value,
Sampling Error is Less than 0.2%
The 32-bit ranges shown above are much wider than the 16-bit ranges but also require the full 32-bit value
to be returned, this requires two sample periods within each scan group. The 32-bit frequency ranges can
also be exceeded with a loss of accuracy or topping out at 4,294,967,295 counts.
Some measurements will require the accuracy of an input waveform to be free of sampling error, having
only the absolute accuracy of the internal timebase as the source of error. Sampling error can be averaged
out to give the required result. In most cases, the WBK17 can perform the required averaging on the values
before they are returned to the PC. The frequency ranges shown below will give a sampling error that is
less than 10ppm or 1ppm.
Full 32-bit Counter <10 ppm Full 32-bit Counter <1 ppm
Range (Hz) Ticksize (nS) Averaging
Option
Range (Hz) Ticksize (nS) Averaging
Option
15u – 500m 20000 1 15u – 50m 20000 1
150u – 5 2000 1 150u – 500m 2000 1
1.5m – 50 200 1 1.5m – 5 200 1
15m – 500 20 1 15m – 50 20 1
150m – 5k 20 10 150m – 500 20 10
1.5 – 50k 20 100 1.5 – 5k 20 100
15 – 500k 20 1000 15 – 50k 20 1000
High Accuracy Frequency Ranges for a 32-bit Value
that has little sampling error (<10ppm, <1ppm)
If the input frequency is required to have less than 1 ppm sampling error and is greater than 50kHz, use the
15-50kHz, 1ppm range. The values returned will have sampling error that is greater than 1ppm but they can
be averaged by the PC software to further reduce the sampling error.