Re: Digital sine wave generation

On 5 Apr 2006 08:06:00 -0700, "Tim Shoppa" <shoppa@xxxxxxxxxxxxxxxxx>
wrote:


The Phantom wrote:
If you have any reference to real designs or published literature that
do DDS-like (e.g. equally spaced steps) or other numerically-controlled
oscillator sine-wave synthesis and do NOT simply choose sin(phase) for
equal phase spacing, I'm very interested. DDS has become very popular
over the past decade or so but none of the published designs or chips
I've seen use anything but sin(phase) for equally-spaced phase in their
lookup table.

The SW4024 family of inverters made by Xantrex
http://www.xantrex.com/web/id/45/p/1/pt/18/product.asp
generates a waveform that is a stepped sine approximation. The amplitude
of the steps varies with load, so the timing and number of steps is varied
to keep the complete waveform approximately 120 VAC. The way the THD
varies with the number of steps is surprising.

Maybe I should've written "that do sine wave synthesis and do a good
job" :-). Admittedly their load is a lot more reactive than mine...

I'm surprised at their published waveform, too. With those big pips at
the top and bottom of each sine wave, it's looking more like a triangle
than a sine! By my calculation that amplitude of that pip should be a
percent or two, and it's looking more like 15 percent in their
published curve!

Where did you find the waveforms (URL)?


In that respect it looks a little bit like the distortion that comes
out of an 8038 etc. (although I know that uses an entirely different
method.)

Tim.

.

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