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Functionality of Rbias pin on CC2530

Michael Marks
Prodigy 325 points
Other Parts Discussed in Thread: CC2530

I have a customer using the CC2530 and is failing FCC in Rx mode around the 2nd harmonic 2.4GHz.  They were able to improve the performance by a couple of dB by adding a 5.6pF cap in parallel with the pull-down resistor (56K ohm) on Rbias pin #30.  They are not sure if this is a valid fix since they do not know what the functionality of Rbias pin is and if a cap will cause problems.  Can you comment?

  • 0
    TI__Prodigy 325 points

    Failing around 2nd harmonic at 4.8GHz not 2.4 GHz in the original post.

  • 0 in reply to Michael Marks
    TI__Guru**** 317180 points

    What they see is VCO leakage. Could you send the gerber files for the layout? Normally it's possible to fix by either use decoupling caps close to some of the IO pins or in some cases a serial L on the IO. How much do they fail by?

  • 0 in reply to TER
    TI__Prodigy 325 points

    They are failing by about 4-5dB.  I'll see if I can get the layout files, but they are hoping to avoid a PCB spin.  If they have to respin they said they would change to Johanson balun design.  I'll let you know after they try some of your suggestions. 

    Is it a problem at add the cap to GND on the Rbias pin?

     

    Thanks.

  • 0 in reply to Michael Marks
    TI__Guru**** 317180 points

    I would like to avoid a cap to the Rbias bin since this lowers the phase margin of an internal opamp. If they fail by 4-5 dB we need to improve by at least ~6 dB and to have some margin. Note that the type of matching network (integrated balun vs discrete) has no impact on this type of issue since the VCO power leaks out on IO pins. Feel free to send me the gerbers offline when you get them.

  • 0 in reply to TER
    Prodigy 30 points

    Understood about the cap on RBIAS. I am waiting on permission from management to send the gerbers to you and will send them as soon as I can.

    Meanwhile, both TDK and Johanson application notes claim that their monolithic baluns reduce spurious emissions, but could that apply to TX mode only?

    Thanks,

    John

  • 0 in reply to John Holt
    TI__Guru**** 317180 points

    Yes, that is typically when you are in Tx. In Rx it's a leakage and the RF signal comes out of the chip via other pins than the RF_P/ RF_N pins so the balun does not impact.

  • 0 in reply to TER
    Prodigy 30 points

    Discussion of the Gerber files was done outside the forum so I am adding the discussion that took place in order to bridge any gaps and perhaps we can continue from here.

    Reply from TER after reviewing layout:

    This looks like a typical VCO leakage layout. The ground plane is splitted by the U shape power routing creating long return paths for the decoupling caps. The IO line are all very long and probably acting as antennas. The get the Rx leakage down you have to relayout I’m afraid.

    Next question posted by JH:

    Thank you for your help. Do you have an example layout you could share? Also, can you list the I/O pins that are the worst radiators?

    This brings us up to date with the postings made so far.  I have more questions but those should wait until we have addressed the current ones.

    Thanks!