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DAC34SH84 DC-Offset Problem

Jin Kyung Han
Expert 1520 points
Other Parts Discussed in Thread: DAC34SH84, DAC3484

Hello

I would like know how to decreas DC offset in ouput of DAC34SH84.

As The below is Block-Diagram with frequency plane, The output of DAC34SH34 has RF signal(Desired signal + DC Offset(Undesired Signal).

I checked the ouput data of FPGA through Capture with FFT, it does not have DC-Offset. Therefore, I reconized that the source of DC-Offset comes from DAC34SH384.

Please let me know how to remove the DC-Offset. 

======================== Below ==============================

FPGA(Base Band: -25MHz-60Khz)--> DAC34SH84(IF: 3* Fs/8 = 552.96MHz, Fs=1474.56MHz)--> IQ Modulator(Loca:2042.1MHz) -->RF ouput : 2570MHz(Desired signal) + 2595.06MHz(DC Offset).

=============================================================

Thanks and Best Regards

 

  • 0
    TI__Guru* 79214 points

    Hello,

    The DAC34SH84's DC offset feature can help adjust the DC offset introduced in the signal chain (both DAC and analog quadrature modulator). Often, DC offset of a few uV to mV range is needed to correct the offset and adjust the LO leakage.

    Please check out the following articles:

    http://www.analog-eetimes.com/en/signal-chain-basics-70-digital-correction-of-analog-quadrature-modulator-imbalances.html?cmp_id=71&news_id=222904086

    section 8.3 of the following:

    http://www.ti.com/lit/ug/slwu029d/slwu029d.pdf

    -Kang

  • 0
    Intellectual 570 points

    Hello Jin,

    I am also working with this module. From your data it seems like I/Q imbalance, not d.c. offset (which appears at carrrier frequency+baseband frequency =2570+25.06= 2595.06 MHz). I enable NCO, and not mixer though.

    Most probably you already know, I/Q imbalance will look like replica of desired signal, while D.C. offset looks like single tone at carrier frequency.

    I would have thought d.c. offset to appear at 2042.1 MHz.

    If it is I/Q imbalance, you have 3 variables in each channel to tune it manually using GUI. For D.C. offset also you have 2 options for each channel to tune offset A, offset B manually.

    As I am also working with same module, I would be interested in knowing if you were able to do it and what performance you are able to get from it.

    Thanks,

    Meenakshi

  • 0 in reply to Meenakshi Rawat
    Expert 1520 points

    Dear
    Mr. Meenakshi

    As you mentioned dc offset
    appear at the local frequency. I agree with you.

    When I use an external mixer,
    your below comments are a wonderful support.

    But now I want to know that
    coarse mixer may generate dc offset in a dac34sh84.

    DAC input data dc-offset
    convert to 552.96MHz and then convert to radio frequency 2595.06MHz.

    External mixer dc-offset
    frequency is 2042.1MHz and coarse mixer dc-offset is 552.96MHz.

    I can cancel 2042.1MHz carrier with RF channel filter but can not decrease 2595.06MHz carrier.

    Best Regards

  • 0 in reply to Jin Kyung Han
    Intellectual 570 points

     

    Hi Jim,

    I am working on 950 MHz with 38.4 MHz IF. These are results for TSW34SH84 which uses DAC34sh84. After using GUI tabs, I am able to remove DC offset at carrier frequency ( shown at 0 in the following figures) and reduce I/Q imbalance shown at -38.4 MHz. Are you receiving similar results? you said you were able to remove carrier at 2042.1MHz, so D.C. offset at carrier frequency is removed I guess and you are dealing with I/Q imbalance product.

    Can you give me your configurations and what you are getting?  I also have DAC34sh84 module separately, which I am planning to start with soon, I can try your settings etc. and then see what seems to be problem.

    It will be learning experience for me too.

    Thanks,

     

     

  • 0 in reply to Meenakshi Rawat
    Expert 1520 points

    Hello

    THe below is configuration and its waveform regarded to DC-Offset.

    Please review it and give me your opinion.

    ================== Below ===============================

    6404.dc_offset_ti_question.ppt

    ======================================================

    Regards

  • 0 in reply to Meenakshi Rawat
    Expert 1520 points

    Dear Mr. Meenakshi

    As you requested me about the configuration for DAC3484, I send you it.

    have you reviewed it? if you did it please let me have it.

    Thanks and Best regards

  • 0 in reply to Jin Kyung Han
    Intellectual 570 points

    Dear Jin,

    The FPGA setup is occupied for another application. could you please allow me 2-3 more days. It should be free from Monday.

    I reviewed it and it looked like D.C. offset but also waveform modulation. Let me replicate it and come back to you.

    If you could add me to your friend list, may be we can avoid non technical discussion like this without posting on forum.

     

    Best Regards,

     

     

  • 0 in reply to Meenakshi Rawat
    Expert 1520 points

    Dear Mr. Meenakshi

    I have another question about operating DAC3484, the I/Q ouput of DAC3484 is imbalance.

    I tested this through that TSW1400 pattern generator linked to the DAC3484EVM. I atatched file it has test procedure and configuration in terms of H/W and S/W.

    8255.TSW1400 - DAC3483 EVB_WTK.ppt

     Please give me solution which point wrong I did.

    Thanks and Best Regards

  • 0 in reply to Jin Kyung Han
    Intellectual 570 points

    Hello Jin,

    I do not use TSW1400 pattern generator. I use FPGA. It looks like that A channel is working fine and problem is in B channel only.

    And also it seems that these extra components are not I/Q imbalance. I/Q imbalance looks like actual signal.

    It will be better if you receive an answer from Ti personnel when using TSW1400 pattern generator.

    Thanks

     

     

  • 0 in reply to Meenakshi Rawat
    Expert 1520 points

    Hello Meenakshi

    Thank you for your answer.