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high attenuation when XMC demodulator card is put in series with the (RF to IF receiver card)

kamlendra chandra
Prodigy 20 points
Other Parts Discussed in Thread: LMH6554, OPA695, TINA-TI

Design : We are having RF to IF receiver card which is interfaced to our XMC demodulator card (snapshot till ADC input ) also pdf 6266.4_PDFsam_1_PDFsam_crl5686_fpgabased_xmc_card_2016.pdfhas been attached for clarity.

Issue : When RF to IF receiver(60mhz)  standalone is used it gives a gain of around 22dbm

             But when XMC card is attached input signal gets a gain of only 15dbm (for further refrence see the snapshot of excel table below)XMC_observation.docx

Please look at the filter stages of XMC card and suggest to improve this degradation.

  • 0
    TI__Genius 15975 points
    Hi Kamlendra,

    Some questions/comments to better understand the issue:
    1. How are you measuring the standalone RF to IF receiver performance; is it using a VNA? Also, what's the resistive load being driven by the RF to IF receiver?
    2. Would it be possible to clarify which filter topology is being used at the OPA695 and the LMH6554? It would be helpful to see the actual schematic with only the populated components being used for debugging purposes, as it is unclear whether the low pass filter or band pass filter is being used.
    3. I would also recommend to populate the R22/R23 and R57/R59 for setting the bias point at the non-inverting input.

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Prodigy 20 points
    Hello,
    For Point 1. I will send you the clarification, as it was measured in Ghaziabad BEL.
    2. It is a Band pass filter 60MHz IF - Chebyeshev 4th Order. Only Filter components required for Bandpass filter are mounted.
    3. We are populating both these resistors on board.
    If required I will send the tina simulation file for the same.
  • 0 in reply to kamlendra chandra
    TI__Genius 15975 points
    Hi Kamlendra,

    Sorry for the delay in reply.

    Could it be possible that you are losing some gain in the band pass filter due to not sufficient Q in the inductors? I compared the gain numbers in TINA-TI for with and without the band pass filter and there seems to be around 4.5dB loss in your circuit with the band pass filter compared to the one without it.

    Also, is the XMC card gain measured at the LMH6554 FDA output or at the ADC input? Please note that you will have 6-dB loss between the FDA and the ADC due to the 100-ohm (diff) series matching resistor and 100-ohm load.

    Best Regards,
    Rohit