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Generating a 100kHz Sine Wave from the AD9834

Question asked by Caboose on Oct 30, 2012
Latest reply on Nov 15, 2012 by musach

Hi all,

 

I have been using the AD9834 for a few weeks now but I have been unable to get it to correctly generate a 100kHz sine wave and I am hoping you can help.

 

First of all, I have attached the schematic, but it is the same as the one suggested in the data sheet. The minor differences are that I have an included an AD5620 on FS_Adjust for amplitude control and I am using a 75MHz MCLK.

 

I am able to program the AD9834 correctly and it will give me the correct frequency and phase. However, the shape of the sine wave is very poor due to the harmonics. I have tried a number of frequencies in this range i.e. 90kHz, 95kHz, 100kHz and 120kHz, but the harmonics have the same effect at each frequency and distort the sine wave. I have looked at the sine wave using the FFT function on the oscilloscope and it shows me that the first major harmonic is at 200kHz, 8.4dB below the 100kHz desired frequency. There is another harmonic at 300kHz, of the same magnitude as this, another harmonic at 400kHz, 3dB below this one and so on (see attached picture).

 

I noted on page 17 of the data sheet that if the ratio between fout to MCLK is small, aliased images may be very prominent, as is the case here. However, Figure 22 on page 12 shows 120kHz being generated using a 50MHz clock and the images are quite small.

 

Is there any way to replicate this figure 22 for my application i.e. ~100kHz sine wave using a 75MHz clock? If not, could you please suggest a way by which I able to generate a 100kHz sine wave using the AD9834?

 

Thank you for all your help.

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