[time-nuts] DDS - Cosine v. Sine LUT
rlajeunesse at sbcglobal.net
Tue Jun 21 19:31:42 UTC 2011
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.
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.
Brent, KD0GLS, Minneapolis
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