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LMX2571: Device immunity versus DC/DC converter

Part Number: LMX2571

Hello,

I have implemented an LMX2571 on a custom PCB, and this one has been placed close to an LTC3119 Buck-Boost converter. This converter is set to work at 500kHz.

Some tests show that while the LTC3119 converter is power on, the LMX2571 have heavily perturbated RF output : peak frenquency is shifted of +10kHz and the spectrum is flattened. When the LTC3119 is off, we have the desired behaviour (frequency stability, constellation, ...)

 

As I guess some internal part (Phase loop, VCO, ...) are pertubated by this DC/DC converter, I would like to know if you have any elements about immunity of the LMX2571, or specific point of attention I may have in the layout using this part.

 Don't hesitate if you need more information/details about the design to offer better answer.

 

Thank you,

Benjamin.

  • Hi Ben,

    Is the DC/DC converter providing power supply to LMX2571 and the reference clock source (e.g TCXO)? If not, then it is a coupling issue. Loop filter and OSCin pins are sensitive to interference. If you use external VCO, then it is also sensitive to interference. Keep these components or pins away from the DC/DC converter.

  • Hi,

    Actually, no, the LTC3119 is used to power the RF power amplifier. The TCXO has its own LDO, as well the LMX2571.

    I don't use an external VCO, but the PI filter of CPout is placed just near that DC/DC converter. So, I'm now almost sure the issue is related to the layout choices.

    Supposing compacity requirement, will adding a shielding box around the LMX2571 and its passives parts may prevent that kind of issue ?

    Thank you for your answer.

    Benjamin.

  • Hi Ben,

    A shield box is always a good idea for a VCO.  Even with a shield box, try not put the VCO close to a DC/DC converter as the shield is not effective in blocking magnetic field.