Running a DDS backwards?
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Yup. As you clock it faster and cross the Nyquist rate, it runs backwards. So the signal phases reverse.
Well, I thought it was cool.
John
So, out of, say, 4096 entries in the lookup table, you're skipping 4095 or
4097 each pass? (Or any other integral fraction of the length.)Tim
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Yup. Suppose you have a 4096 point waveform table, and the system clock is 128 MHz. If the phase accumulator value is, say, 7FF00000, you're skipping 2047 points every clock, and the sinewave is just a hair under Nyquist, 63.9... MHz. At 80000000, the output is zero. At
80100000, you're skipping 2049 points every clock, equivalent to-2047, so you're over Nyquist and walking the table backwards.
John
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