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

[KD0GLS]

That's an interesting thought.  The diagram of the 9835 (the one labeled as having the cosine ROM) also shows some sync logic associated with the select lines steering the FSK and PSK registers.  If that logic syncs the select lines to the phase accumulator rollover, as you said, the slope of the signal would be near zero at that point, as opposed to at the zero-crossing with a sine-based device.  An interesting theory at least.

On Jun 21, 2011, at 14:31, Robert LaJeunesse wrote:

> Brent,
>  
> For the specific case of generating a synchronous FSK signal with a fairly wide shift there may be a reason. Such an application presumes a high enough ratio between clock and output frequencies such that the DDS accumulator landing adequately near zero is a certainty. If the FSK frequency is changed synchronously - just after the point of DDS accumulator rollover - a sine LUT would potentially show an abrupt change in dv/dt (slew rate) with the frequency change. By using a cosine LUT the signal would be at its peak, and dv/dt would be virtually zero both before and after the frequency change.
>  
> Bob LaJeunesse
> Ann Arbor, MI
> 
> From: KD0GLS <kd0gls at mninter.net>
> To: Time-Nuts <time-nuts at febo.com>
> Sent: Tue, June 21, 2011 1:44:26 PM
> Subject: [time-nuts] DDS - Cosine v. Sine LUT
> 
> With all the discussion lately regarding DDS and CORDIC, I'm reminded of a question that came up some time ago for which I've never found an answer.  Perhaps you enlightened people can enlighten me.
> 
> Given a complete DDS chip with a single output channel (e.g. AD9834, AD9835), why would one device favor a cosine LUT versus a sine LUT?  On the surface, starting the roller coaster ride at the top of the hill (assuming the phase accumulator starts from zero) seems odd.
> 
> .73,
> Brent, KD0GLS, Minneapolis
> 
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.73,
Brent, KD0GLS, Minneapolis