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AD9850 Frequency Modulation

Question asked by jomac on Dec 29, 2013
Latest reply on Dec 30, 2013 by jomac

We have a requirement to modulate the output of the AD9850 by frequency modulation and wondered if anyone had any experience or advice in this area?

 

We notice that the 9850 can have its frequency changed at high speed, but this is not really a path we want to go down and prefer to stick with something simpler, and preferably analog.

 

We wondered if we could actually modulate the 125Mhz clock input in order to obtain an FM output. What we are aiming for is about 10Khz deviation at 28Mhz, and the 2 questions we have are, A, can it be done this way, and B, If it can, is the deviation amount halved each time you half the output frequency? ie if the clock was FM'd at 10Khz, the outputs at 62.5 and 31.35Mhz would they have deviation at 10Khz or 5 and 2.5Mhz respectively.

 

If this is the case, we would be approaching a deviation value of well over 40Khz at 125Mhz clock to get 10Khz at 28Mhz, would this device tolerate this?

 

Our only other option would be to mix the output to a different frequency and modulate the mixing osc, then mix back to the original with add FM but this seems a long winded way of doing things.

 

Can anyone throw in any suggestions here?

 

John

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