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TRF3720 Demo Kit with OFDM issue

Steven Lee
Prodigy 20 points

Hi Sir:

An issue in DTV Band of OFMD modulator from DAC output  "DVB-C" mode to TRF3720 REV.C demo kit.

RF detect constellation as MER 29~30dB only in the QAM256. (These Zero-LO mode.)

When I tries IF mode by 6Mhz out band. Then MER up to 38dB in the DVB-C, QAM256 too.

Next lab: I try mode DVB-T mode and verifies again. MER is 38dB in QAM64, DVB-T.

The DAC output is 45MER to IQ input of TRF3720 demo kit(Rev.C).

Also, I had confirm symbol size as band width from 5120, 6120, 6900 ...etc. It's same MER 29~30dB.

And trim balance between LO with IQ offset. Then LO at -65dBm under.

But DVB-T is good quality on demo kit. 

Why is not good on Zero-Lo at DVB-C mode??

  • 0
    TI__Mastermind 18065 points
    You are likely seeing issue related to the following:

    - At ZIF, LO feed-through impacts performance. You an adjust DC offset voltage to minimize this contribution.
    - At ZIF, sideband suppression performance impacts performance since the image is right on top of the desired signal. Adjusting amplitude and phase balance on the BB inputs can minimize this contribution.
    - If LO frequency is above 2.4 GHz, there is some isolation issue inside the device that manifests as HD2. At ZIF, the HD2 will encompass the desired signal and degrade the performance. Operating less than 2.4GHz engages dividers and the isolation improves.

    All of these impairments impact ZIF applications. At a low IF approach, these impairments exist but they are not on top of your signal and hence the MER is improved.

    --RJH