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Ripple on output waveform from DDS

Question asked by BenT on Jun 11, 2012
Latest reply on Jun 18, 2012 by Joyride

Hi there,

I am new to DDS application and not sure if DDS is right way to achieve what we need.

 

My goal is to first provide a sweeping output from 13MHz to 14MHz at 1KHz step at the output of an interrogator.

The purpose is to find the exact resonant frequency of a RFID alike antenna within this band, and then

the interrogator would lock into this frequency to talk to this antenna ( and the device behand it). This is to be

done in a controlled environment, therefore FCC regulation is not a concern at this time.

 

I have an EVAL-AD9834SDZ evaluation board running in the lab and I am a bit surprised to see, on scope,  ouput

waveforms near 13MHz all have noticable sawtooth on the trace (scope_1). After limiting scope probe bandwidth,

output traces look smoother by still have decernable ripples(scope_0).  Please confirm these scope captures are

typical of DDS outputs.

 

With these ripples present (after low pass filter on the scope), my frequency counter reading varies +- 100KHz

when I set DDS output at fixed 11MHz ( for example). Does this tell me that I could not achieve a freq resolution

anything below 200KHz because of the ripple, dispite the fact that I have a 28 bit register on AD9834?

 

If AD9834 can not give me the control on resolution of 1KHz in eval board, could increasing the master clock freq help ?

Any solutions or other ICs you can suggest?

 

Thanks for your help,

Ben

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