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

[Robert LaJeunesse]

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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