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DAC3482 EVM peak power in higher nyquist zone operation

Florence Zhu
Intellectual 310 points
Other Parts Discussed in Thread: DAC3482

Hi,

I am testing the signal output power in higher nyquist zone and get different result from what theory predicts as I understood it and wondering why.

If I model the sinc function power spectral density in matlab, the 2nd lobe peak power ratio to main lobe is 13.3dB down, and 3rd lobe peak power is 17.8dB down from the main lobe. shown below:

When I program the DAC3482 EVM with following setup, I got the 2nd lobe about 11dB down and 3rd lobe signal also about 10dB down from main lobe power. (shown below) I am not sure why the actual prediction is a bit off from theory, what other factor goes into it. I also experimented with different datarate, each time the result is bit different, and not all that close to theory prediction. Thanks,

DataRate=245.76MHz

DACCLK=245.76MHz

Interpoloation=1

IF = 1Mhz (reference main lobe peak power), 61Mhz (to generate signal close to the peak of the 2nd lobe and 3rd lobe)

  • 0
    TI__Guru 71959 points
    Florence,

    The DAC348x family is designed to operate in 1st Nyquist zone only. With higher Nyquist zone operation the DAC performance degrades, and you will observe SNR and SFDR performance degradations.

    Ideally, the DAC output should follow the sinc response that you have plotted, assuming that there are no other analog imperfections. One main contribution to the mismatch of the prediction is the VSWR of the DAC output. The DAC output on the EVM is transformer coupled with 4:1 transformer , transforming the DAC's 200ohm resistor ladder network on the primary to 50ohm secondary. This allow easy interface to 50ohm based equipment such as your spectrum analyzer.

    The transformer used is Mini-circuits's ADT4-1WT, and you can see the return loss drops below the 10dB rule of thumb after about 600MHz. This works well for DAC348x's 1st Nyquist zone, but may cause gain flatness issue for output frequencies above that. You can try to add some attenuator pads (SMA based) to try to improve the return loss/gain flatness. However, do keep in mind that these are high Nyquist zone replicas that you may need to filter out with an anti-aliasing filter anyways.

    -Kang.
  • 0 in reply to Kang Hsia
    Intellectual 310 points

    Thanks Kang for the thorough explanation, very helpful and I really appreciate it!

    Florence