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CC1310: Transmission images at 24MHz clock multiples

brendan westhorpe
Prodigy 60 points
Part Number: CC1310
Other Parts Discussed in Thread: CC1190

I have tested on our custom hardware and on multiple launchpads and have found there is modulated images at +-24MHz, +-48MHz, +-72MHz, +-96MHz etc that are mixing products from the 24MHz clock transmission internal to the CC1310. The worst of these is -96MHz from the fundamental and causes issues with New Zealand certification (with 20dB amplification in line) in the 915 to 928MHz band due to a restricted band from 800 to 915MHz having a -49dBm limit in 100KHz . This was tested using RFstudio version 2.4.2 with standard setup, constant TX and un-modulated carrier to measure directly into a spectrum analyser (after capacitor moved and cable added). The worst level of image was at 6dBm CC1310 output power measured at -72.48dBm but with inconsistent level, it improves above and below this and is at least 13dB better at 12.5dBm output (hit measurement noise floor).

Could someone at TI shed some light on this issue and any way to reduce the level? I am worried about inconsistencies in this level causing failures at certification. I have more information about test setup/results if needed.

  • 0
    TI__Guru**** 317180 points
    When we have looked at spurs we have used max output power since this typically potentially cause issues with regulations when using CC1310 without external PA.

    On CC1310 + CC1190 a SAW filter is used to avoid issues with phase noise in ETSI and FCC restricted bands. The SAW filter then attenuate spurs. I think we have seen the behavior on a design with a Skyworks PA due to vacation time I haven't been able to get a comment from the engineer in question.

    As I understand your question you can get your design through regulations if you use other settings than 6 dBm on CC1310? If that is the case, is that an issue?
  • 0 in reply to TER
    Prodigy 60 points
    My question is about the mechanism and what the maximum levels of this spur can be, as at the moment I have no indication of what the level can be only measurements from a few units. There could be a unit with 10dB higher level for example which would cause a failure. When I know that I can design a filter to reduce the level.
  • 0
    TI__Mastermind 29570 points
    Which PA are you using?
  • 0 in reply to RGW
    Prodigy 60 points
    We are using the SE2435L as a PA.
  • 0 in reply to brendan westhorpe
    TI__Mastermind 29570 points

    Can you send me your schematic ?

    Send as a private message if you do not want the schematic to be public.

    //Richard

  • 0 in reply to RGW
    Prodigy 60 points
    Hi Richard,
    Is there any update with this issue, it looks to be linked with PA bias.
    (-85dBm noise floor on measurement)
    CC1310 power 819.2MHz 832MHz
    -10 -85 -85
    0 -76.9 -77.5
    1 -76.43 -77.3
    2 -74.08 -75.4
    3 -75 -75.3
    4 -74 -74.2
    5 -72.97 -74.58
    6 -72.5 -74.07
    7 -72.48 -73.38
    8 -73.44 -74.65
    9 -73.88 -74.56
    10 -76.7 -79.26
    11 -81.7 -83.2
    12 -85 -85
    12.5 -85 -85
    14 -85 -85