hi

I'm using AD9833 and only need square wave output.

but square wave out of AD9833 have shaking noise

It outputs very clean square wave, MCLK/2, MCLK/4, MCLK/6...... but other frequency have noise.

Please see attatched picture.

6.25MHz, 2.5Mhz .. OK

8.00MHz, 2.10MHz have noise (for example)

Any possible solution?

Hi,

The square wave output of AD9833 is taken from the MSB of the phase accumulator.

Actually, what you are seeing in the scope is not noise. This is the effect of the phase accumulator output when the frequency output set is not an exact multiple of MCLK.

To further explain:

The formula for Fout = (MCLK/2^28) * FREQREG

Rearranging: 2^28/FREQREG = MCLK/Fout

A ratio of multiple of 2 must be satisfied such that the midscale value is exact on each roll over of the phase accumulator.

If the FREQREG is not an exact multiple of 2, then there would be a remainder on each roll over. The remainder differs from one over the other. Thus, the effect is different midscale (MSB=1) on each roll over of the phase accumulation process. Since this is phase, the effect at the output is horizontal jitter --> and that is what you are seeing on 2.1MHz, 2.5MHz and 8MHz output.

If you want to have a less-jitter output, you should have an output that is an exact 2^n multiple of MCLK (eg. MCLK/2, MCLK/4, etc). Or you can use an external comparator and use AD9833 to have sinewave output. Or, you may use AD9834 instead. It has an internal comparator for square wave output.

Hope this helps.

Best regards,

Sittie

added examples for fout Message was edited by: sittie aisha magayo-ong