# what if the frequency of the DDS output signal we want is the same as the image signals?

Hi, I am Ye-Eun Chi.

I have a DDS of AD9959.

That  has limitation 500MHz as reference clock frequency.

If I generate signal 250MHz as DDS output, the frequency of image signal is same(250MHz).

Can I use this signal?

What happens if I control the phase at this time?

If I shift 1 degree of 250MHz, Is the DDS output signal shifted 1 degree?

I would really appreciate it if you give me an answer.

Thank you.

• Hi Ye-Eun Chi,

Thank you for using our product. I'm not sure on how you would use the output signal of the DDS but I would suggest to use an LPF to filter out the image frequency and minimize the distortion. However if you prefer to retain the distorotion due to the image frequencies, then an LPF is not needed. You can still control the phase at that configuration where 250 MHz is the frequency of the sinusoid. You can attempt to detect if there is a phase change by using one of the 4 channels of ad9959 as reference (0 deg phase) and the other one to have a one deg phase offset.

Best Regards

Louijie

• With a 500MHz sample clock, programming the DDS for 250MHz is equivalent to generating a DC signal. That is, no tone, but a constant output current from the DAC. Essentially, the DDS is repeatedly generating an amplitude associated with the same sample point on a sine wave over and over again. Hence, if you change the phase, the sample point on the sine wave changes resulting in a DC level change.

Operating in this mode is similar to using codes to change the output level of a DAC. Normally a DAC responds to input codes linearly. However, in this case, the DAC responds to input codes in sinusoidal fashion. That is, by producing an output current based on sin(2πx/2^14), where x is the 14-bit value you program into the phase register.