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Problem with amplitude of DDS output

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Ricewind

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Hello,

i am currently using a AD9959 DDS in order to generate various control senoids for a analogic circuit. The problem I find is that the output amplitude varies depending on the frequency I set. For example, I get 2.85 Vpp at 55 MHz and 554 mVpp at 20 MHz.

I know I can make the output amplitude lower through OSF control and thus having the output at similar voltages... the problem at this is that the voltage will be the smallest one of the ones I can get and it is not enough to operate the following circuit without an intermediate amplifying stage.

I do not understand why these different amplitudes come out, as the DAC should be able to generate signals with the maximum amplitude for all the frequency range, so the problem will be likely to be internal to the digital stage of the DDS.

Any help at this?

Thank you in advance.
 

The chip is certainly capable of putting out both signals at the same amplituded, so the doubt is either in how you are programming it, how you set up the chip with external hardware, and how you are measuring it.

Are you using ADI software to program it on their demo board, or is it your custom software on your board?

What is the clock frequency the DDS is using? You will need at least a 150 MHz clock.

Are you using an output lowpass filter, and if so what is its cuttoff frequency. How many db of attenuation does it have at the alias of 55 MHz?

How are you measuring the output? Analog oscilloscope or digital oscilloscope? Is your sample rate high enough to avoid scope aliasing? Does your analog lowpass filter elimininate the chance that the oscilloscope is seeing an aliased frequency?
 

At now I am just using the debuging software that ADI provides in an evaluation board.

I use a 27 MHz oscillator with a multiplying factor of 18, thus the clocking signal is 486 MHz.

I have already seen the possibility of being the lowpass filter the responsible of such behavior but it does not seem so. I read the signal both before and after the filtering stage and the variation is the same in both of the cases.

It is a bit strange because the amplitude varies in a decresing trend from 1 MHz until 30 MHz, then increases until a peak in 55 Mhz and continues decresing from here on.

I measure the output with an analogic oscilloscope. Though I do not have the specifications, it is normally used for high frequence analysis... so i think it should fit.

Thank you very much for your answer.
 

I do not understand why these different amplitudes come out, as the DAC should be able to generate signals with the maximum amplitude for all the frequency range, so the problem will be likely to be internal to the digital stage of the DDS.
The DDS won't apply frequency dependent amplitude variations on it's own. I see two possible explanations:
- You have caused it unintentionally by an incorrect configuration
- The problem is generated in your analog hardware
 

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