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The reference frequency was stepped in 10 KHz increments from 110 to 300 KHz approximately every 10 seconds. The GPIO cards were configured to count the number of tics from the onboard 20 MHz clock while counting 10,000 cycles of the reference input (which is the same manner that the frequencies of the ParoScientific transducers are sampled). These counts are repeated every 0.1 seconds. The resulting frequency measurements were then compared against the reference frequencies.
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