software DDS

geolover · Joined: 08 Feb 2011 Posts: 18

Hello!

I want to implement a software DDS with a PIC and a DAC chip.

I know the standard procedure involves generate phase first then put phase into a sin function.

However, when trying to generate a 480 points sine wave look up table of 500Hz, I began to experiencing problem: I found a lot of numbers in the sine wave table are repeative and therefore useless!

This is the following equation I used to generate the sine wave table:

Phase=2*pi*f0*i/N;
f0 = 500;
i= from 0 to 479;

N=480

sample = 2^14+(2^14 - 1)*sin(phase);

because the DAC is 16-bit and its largest positive representation is 0x7FFF.

Also, could anyone explains to me about the relation of digital output rate, number of points in the look up table, and the frequency of synthesized waveform ?

temtronic · Posted: Mon Mar 14, 2011 3:52 pm

If the 480 points is over one sine wave, you really only need to calculate 120 points( first 90*)...
it's a 'cheat' I found on the web called 'magic sinewaves'.....years ago...

geolover · Joined: 08 Feb 2011 Posts: 18

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

The following CCS example files use a lookup table to create a sine wave:

newguy · Joined: 24 Jun 2004 Posts: 1903

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

With any DDS work I have done the number of points in a full cycle is always a power of 2, generally 256 for crude 8-bit stuff, or 2^16 for fine (but slow)16-bit stuff. It is basically the length of the phase register. That way when the phase register rolls over you simply start on the next cycle. The resulting frequency is therefore the DDS clock speed * phase increment / phase register size.
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geolover · Joined: 08 Feb 2011 Posts: 18

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

If you only update the phase register every 2ms, the output state will only change 500 times each second. Frequencies over 500 Hz are clearly not possible. Actually Nyquist sampling tells you you can only get a theoretical maximum of 250Hz with a 2ms update cycle.

I generally start with the max frequency I need to generate. I need my DDS cycle to be 2.5 or 3 times that fast. Then I see how wide a phase register I can update at that speed. The DDS work I have done has all been square wave output. But you need to see if you get the sine amplitude resolution you need with the width of phase register you can update.
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SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

Here is another way to look at it. Your sine wave is going to be built of little stair step blocks. Decide how long, in time, each of these blocks can be. That is how long you have to update your phase register, do your sine lookup, and load your DAC.

If you only have 20 blocks in a full cycle, that is 5 steps up and 5 down for each half cycle, and each cycle is 10ms (100Hz), you need to run your DDS loop every 0.5ms. If you can only loop your software every 2ms you either limit yourself to 25Hz output, or you make your blocks bigger and your sine wave gets really ragged.
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The search for better is endless. Instead simply find very good and get the job done.