Re: Obscure Electronics Topics

On Mon, 26 Nov 2007 03:57:22 -0600, "Tim Williams"
<tmoranwms@xxxxxxxxx> wrote:

"John Larkin" <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:p2jkk3lum25uelsbpbsrgm6uc63f9aokhm@xxxxxxxxxx
Running a DDS backwards?

Yup. As you clock it faster and cross the Nyquist rate, it runs
backwards. So the signal phases reverse.

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

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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