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 <[email protected]> To: Time-Nuts <[email protected]> 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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
