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Spurious when FS/F is not an integer

Question asked by LoganathanN on Jan 24, 2017
Latest reply on Feb 19, 2017 by Dr_joel

Hi all,

 

We are using a 16bit DAC for a waveform generation between 0 to 500MHz with the sampling frequency of 1200MHz. The specification for the DAC performance is >60dB SFDR. The spectral performance of the DAC is fine (more than 65dB SFDR) for the frequencies which is satisfying the condition,

 

Fs/F = Integer

 

The frequencies which is not satisfying the above condition yields some spurious which is not meeting my SFDR spec. Since the samples of every cycle will not be same, it's creating an envelope over the waveform, due to a phase values repeating itself periodically over a no. of cycle. This envelope causing a spurious. If we generate a frequency of 200.01MHz in MATLAB with 1.2GHz Fs, we can clearly notice the Envelope with periodic time period of 16.65us (60KHz).

The below figures are the MATLAB Time domain plot for 200.01MHz & respective Phase.

Time domain plot for 200.01MHz

Phase values in rad for generating 200.01MHz

With my DAC, I'm getting a spurious at + or - 30KHz apart from my generated frequency(F - 200.01MHz).

The below figure is the spectrum analyzer image for the frequency 200.01MHz.

 

Is the spurious observed in the spectrum analyzer is due to the envelope observed in the time domain?

 

Thanks & Regards

 

Loganathan N

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