This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

CC1121: Spurious Emissions

Part Number: CC1121

I am having significant spurious emissions in the 790 MHz - 798 MHz range while operating the CC1121 in the 902 MHz - 928 MHz range.  The circuit uses a 32 MHz crystal.  The datasheet mentions that transmissions at frequencies within 1 MHz of a multiple of the crystal frequency should be evaluated for spurs.  Are the 7xx MHz spurs related to this note?  What is the relationship between the spur frequency and the transmission frequency?

  • Hi,

    To make it easier to look into this on our end:

    • Is this using a custom board or a TI LaunchPad/EM?
    • Please could you provide:
      • A plot of your spectrum.
      • Your PHY settings (could you replicate them using SmartRF Studio if you are using a EM)?
      • Your measurement setup (the measurement settings, ect.)?


  • This is a custom board in volume production and has been through compliance testing (and passed).  I can't provide a spectrum since I don't have a real-time SA and the transmissions are frequency hopping.  My sweeping SA isn't fast enough to get both the 7xx MHz and the 9xx MHz at the same time.  I can look a the  zero-span trace at 7xx MHz  and see an amplitude pattern that corresponds to that of the 9xx MHz fundamental.

    I cannot comment beyond that since I am under NDA to the owner of the design.  Thanks!


  • I would advice to run a seperate FW on the MCU to set the CC1121 in cont. TX on a selected frequency and hence be able to provide a spectrum.

  • Thank you for that suggestion.  I do not have that capability. 

    My question is simple:  Why does the datasheet say that spurs should be evaluated when operating within 1 MHz of multiples of the clock frequency?

  • I would guess that the spurs you are seeing is fundamental frequency - n*32 MHz but without seeing a plot with markers it's difficult to say for sure.

    Most RF chips on the market have clock spurs. Normally they are below the regulatory limit and hence you don't need to worry about them. It's not clear if that is the case here, the first thing you have to check if the spurs are above or below the limits. If the spurs are close or above the limit I would assume that the decoupling on the xosc power is not ideal, most likely a layout issue.

  • I am not seeing pure harmonics of the crystal, I am seeing the modulated transmit signal in the 700 MHz band as though it is being mixed up or down to that range.  Without knowing the structure of the CC1121 transmit chain it is impossible for me to know why the modulated signal is showing up in the 700 MHz band.  I am open to enlightenment.

  • Without having exact numbers its hard to help. 

    - Does the spur change frequency as a function of the fundamental frequency? If that is the case, what is the delta between the centre of the spur and the fundamental?

    - If not (constant spur): Which frequency?