[time-nuts] DDS - Cosine v. Sine LUT

Magnus Danielson 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.

Cheers,
Magnus



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