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.

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

If you have a matlab sim of the signal, it is an easy thing to pass it through the matlab fft function and see what matlab thinks the spectrum is. But surely, if you have something like 10% envelope modulation on your signal, you must see some evidence of that in the frequency domain, so I would suggest it is entirely due to your DAC. Your envelopes repeats every 16.66 usec, which has a 60 kHz frequency, but if the repletion is from envelope modulation a 30 kHz side-band would certainly be expected.