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I am trying to design a sinusoidal signal generator using AD9954 which is a Direct Digital Synthesis (DDS) with takes a maximum of 400 MHz crystal oscillator as reference frequency. It also has a register that can be written for multiplication so that a lower crystal oscillator at 20 MHz for example, can then be multiplied 20 times to get a 400 MHz reference frequency. My Question - is it better to use a crystal oscillator that provides 400 MHz or use a lower frequency such as 20 MHz and then multiply it? Which one would be more efficient? I found a few posts on here such as this but tit did not answer my question.

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    \$\begingroup\$ Datasheet says directly providing the clock has lower phase noise than multiplying it. Using a lower frequency crystal probably gives you a lot more options. \$\endgroup\$ Commented Aug 21, 2020 at 4:30

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My Question - is it better to use a crystal oscillator that provides 400 MHz or use a lower frequency such as 20 MHz and then multiply it?

It depends what you mean by 'better', and what you're going to use this synthesiser for.

Both routes have advantages, and what's important to you will dictate which advantages are more important to you.

Using the multiplication route has the advantage of cheaper components and more flexibility. The fact that you're asking the question probably means that this is the way for you to go.

If you want the best phase noise and spurious performance, say you're designing a high performance professional piece of equipment for sale, then you'd use the external high frequency clock source. If that was one of your considerations, you would probably know up front.

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