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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 cycles. These counts are repeated every 0.1 seconds. The resulting frequency measurements were then compared against the reference frequencies.
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The slope of the line is a correction factor for the 20 MHz clock on the GPIO-MM:
corrected clock frequency = 20 MHz / (1 + slope)
The upper right plot is the measurement error after applying the corrected on-board clock rate to the data.
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