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LMX2594: DC blocking caps required?

Dennis Yuan
Prodigy 20 points

Part Number: LMX2594

Hi,

I am using the LMX2594 differential outputs with a balun to boost the power output. In this case, are dc blocking caps required after the pullup resistors? If so, this leaves the balun floating and I would need to place a large impedance to ground the DC potential.

Thanks,

Dennis

  • 0
    TI__Mastermind 18070 points
    Hi Dennis,

    I don't quite get the part where you said "leaves the balun floating". Could you be more specific about it?

    In general we recommend using AC coupling because the common mode of RF output is poorly defined. It depends upon OUTx_PWR, value of pull-up resistor, as well as output frequency. So it is suggested to use DC block and redefine common mode externally if it is needed.

    Also, it is not common to use external balun at 10+ GHz because at that high frequency, the insertion loss of the balun is often higher than or close to 3dB and it's not worth it to use a balun to boost output power.

    Regards,
    Hao
  • 0 in reply to Hao Z
    Prodigy 20 points
    Hi Hao,

    Thanks for the reply. What I mean is that inserting the dc blocks leave the dc potential of the node afterwards in an unknown state. Forget the balun, say afterwards is an amplifier and I am using the LMX2594 in single ended mode. The amplifier is AC coupled, so now there is a trace that has an undefined DC voltage after the dc block and before the amplifier. Do I still need the blocking cap? What would be the impact of not including a blocking cap?
    Thanks,

    Dennis
  • 0 in reply to Dennis Yuan
    TI__Mastermind 18070 points
    Hi Dennis,

    I'm afraid I still don't understand why DC potential is unknown. The DC component is defined with respect to ground right at the output of LMX2594. The AC coupling capacitor then takes out the DC component so the output swing is now centered at 0 (ground). There can be crosstalk or coupling along the way but essentially the DC potential is 0.

    The reason why we recommend AC coupling is that the DC component at the RF output (common mode voltage) is not well characterized. For example, when the output frequency is 4GHz, the common mode voltage is 1.85V (just an example, not a real number). But when output frequency is 8GHz, it becomes 2V. The same is true with the register OUTx_PWR. So it is good practice to use AC coupling then define DC voltage afterwards.

    However, if you already have AC coupling at the other end (say input of next stage of amplifier) then it should be ok to remove the cap at the output of LMX2594.

    Regards,
    Hao
  • 0 in reply to Hao Z
    TI__Mastermind 18070 points
    Hi Dennis,

    I just realized that voltage between two caps is undefined. You were right about the voltage being floating. Still, only one cap is needed in the path of RFout.

    Regards,
    Hao