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CC2500 2nd Harmonic problem

Cory Duce
Prodigy 35 points
Other Parts Discussed in Thread: CC2500

We're working on a design using a MSP430 and CC2500. We are finding that the 2nd harmonic of the RF is present when the radio is in Rx mode. As a result we are not passing EN 300 440, or FCC 15.209. I've confirmed that the second harmic is being radiated from the board, and is not coming out the antenna. I've compared severalversions of our hardware, as well as a TI reference design, and all of them radiate the 2nd harmonic when in Rx mode. Has anyone else seen this 2nd harmonic problem?


Thanks,

Cory

  • 0
    TI__Guru**** 317180 points

    It is a know issue for some designs. First try is to place 2.4pF caps as close to the power pins as possible.

    Is it possible to post the layout?

  • 0 in reply to TER
    Prodigy 35 points

    I've placed 3.0pF caps on each of the power supply pins right next to the CC2500, I've also experimented with other values but none have changed the 2nd harmonic level. I can't post the layout, but I can say that it is a 4 layer board with solid ground under the RF section, very short power supply traces, very short SPI traces to the radio, and lots of power supply filter placed very close to each of the power pins.

    We've talked to a number of test labs, and RF experts, and all were quite supprised that the radio would be radiating the 2nd harmonic when in Rx mode. Is it possible that there are some register settings that we have wrong that could cause this?

    When I probe around with the spectrum analyser and a small RF loop, the hot spot  for the 2nd harmonic is right on top of the CC2500, and is about the same in both Tx and Rx modes.

    Cory

  • 0 in reply to Cory Duce
    TI__Guru**** 317180 points

    If you send a friend request it is possible to send me the layout one to one after I have accepted it.

  • 0 in reply to TER
    Prodigy 25 points

    We also have the same problem in RX mode. Could you suggest something or review layout?  

  • 0 in reply to Davy Huang
    TI__Guru**** 317180 points

    Send me the layout as gerber offline and I can take a quick look at it.

  • 0 in reply to Davy Huang
    Prodigy 35 points

    Davy,


    We were able to talk with a TI engineer that worked on the design of this chip and he was able to point us in the right direction. The problem is the internal VCO (which runs at 2x the RF frequency) just leaks. The VCO is located on the corner of the die next to pin 15 and 16. Pins 14 and 15 (AVDD3 and AVDD4) supply power to the VCO, and are the main path for conducted emissions to get out of the chip. Bypass these two pins close to the chip. There is a good amount of energy that is just radiated right out of the top of the chip, so it might be a good idea to plan for a shield can. Hope this helps.


    Cory

  • 0 in reply to Cory Duce
    Prodigy 25 points

    Hi Cory,

    Thanks a lot ! It is the same problem on our CC2500 module.  Is it possible to solve by relayout PCB ?

  • 0 in reply to Davy Huang
    TI__Mastermind 18780 points

    Yes. The VCO leakage level is very layout dependent and good grounding is very important to get the design to pass the regulatory requirements. 

  • 0 in reply to TER
    Intellectual 375 points

    Hi TER,

    We are experiencing the same issue in our design.  Can you send me a friend request so that I may send you the layout?

    Thanks