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

[Gerhard Hoffmann]

Am 21.06.2011 21:03, schrieb KD0GLS:
>> 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.

It can be the same table, and if it delivers sine or cosine is 
determined only
whether you start indexing from the highest or lowest address .
(if you store only 1/4 wave)

When you split the ROM into 2 smaller ones, you can get sin and
cos at the same time, there will never be an address clash.
One ROM for 0...44.9 deg and one for 45...89.999° . Just a MUX
to select the appropriate ROM for each output and a little bit of
address massage.

Big cost saver for complex converters.


Gerhard, dk4xp