[time-nuts] DDS - Cosine v. Sine LUT
magnus at rubidium.dyndns.org
Tue Jun 21 19:26:01 UTC 2011
On 06/21/2011 09:03 PM, KD0GLS wrote:
>> On Jun 21, 2011, at 13:39, Chris Albertson wrote:
>> I used only 90 degrees of the table.
> Yes, as did I and most implementations, but why a cosine quarter-table instead of the more common sine? A quick look at the data sheets (and the waveforms in the theory-of-op sections) for the two devices suggests they are clearly calling out one or the other, but without rationale regarding the choice.
By the way, you can save two bits of the LUT table width by only storing
the difference of the binary phase and the sine. Add the phase
(90-degree wrapped) to the output of the LUT to get the sine. In effect
the LUT can be allowed to be shifted two bits down for improved precision.
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