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CDCM7005: Interfacing with LVCMOS VCXO.

Filip Ozimek
Prodigy 20 points
Part Number: CDCM7005


We want to use CDCM7005 to improve clock jitter. The evaluation board (CDCM7005-SP Evaluation Module) is configured to work with PECL VCOs although we plan to use LVCMOS 100 MHz VCXO. The board is populated with PECL terminating resistors (130R to Vcc and 82R to GND) for both VCXO_IN and VCXO_IN_N inputs and there is a pin providing VBB  to bias unused complementary input.

My question is: what is the proper way of interfacing LVCMOS VCXOs to CDCM7005?

I plan to:

- remove terminating resistors from PCB at VCXO_IN_N input (R31, R36). Connect input to VBB bias volatege via jumper (R28).

- remove terminating resistors from PCB at VCXO_IN input (R38, R43), remove AC-coupling (C17 - jumer 0R).

Please share your comments.

Kind regards,

F.

 

  • 0
    TI__Expert 8865 points
    Hi Filip,
    Thanks for posting your question. I have assigned your request to an expert and he will get back to you very soon.
    Best regards
    Puneet
  • 0
    TI__Expert 5370 points
    Dear Filip,

    This is a good question. I suspect you will want a voltage divide on the LVCMOS to get to move +/- 800mV as PECL does and set common mode to 2V on each leg. Let me check with a team member and we will get back to you.

    thank you for your patience.

    Regards, Simon.
  • 0 in reply to Simon Damphousse
    TI__Expert 5325 points

    Hello Filip,

    here a visual clarification for Simon's suggestion.

    • Ensure the signal swing is lowered below < +/- 800 mV at the pin
    • The complementary pin !VCXO_IN shall be biased to VCC - 1.3 V. You can use the VBB pin for convenience. We recommend to place a decoupling cap on the VBB to keep it stable.
    • The attenuated VCXO signal should be AC coupled. You can use 100 nF as a start value.
    • The AC-coupled and attenuated signal common mode should be close to the threshold voltage via VBB. You can either keep the pull-up/down termination or terminate with a resistor to VBB. The values depend on the drive capability of your VCXO.

    Best regards,

    Patrick