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CC1200EMK-868-930: Receiver receive unwanted frequencies

Ian Def
Prodigy 60 points
Part Number: CC1200EMK-868-930
Other Parts Discussed in Thread: CC1200

I am using CC1200 only as receiver (863-869MHz and 903 - 927MHz bands) and I use SmartStudio to setup registers.

I noticed that receiver can receive packets from small distance of transmitter (500mW) at wrong frequencies which are +40Mhz or -40Mhz.

What I want to say is that if transmitter sends packages at 863, I can receive it if receiver is set at 863 (expected) but also if I set it to 903Mhz (only a few meters from transmitter) and vice versa.

I have checked transmitter with spectrum analyzer and I can confirm there is no harmonics at +-40Mhz.

I have tried to find another distance instead 40MHz but didn't find. What is happening? That means that I will have trouble if someone is noising +-40Mhz from my central frequency.

  • 0
    TI__Mastermind 20210 points

    Hi,

    That's quite odd.

    Are you testing the receiver with SmartRF studio? If not, can you do the RX test with SmartRF studio and share the settings?

    At +/-40MHz offsets do you see packets without CRC errors?

    Regards,

  • 0 in reply to FI
    TI__Guru**** 317180 points

    I don't think we have done a similar test but I note that from test results I found that the sensitivity is poor at multiple of the xtal frequency. I suspect that a signal that is present at a multiple of the xtal frequency will be mixed down to the main channel. 

    Are you using address filtering to avoid processing packets that are sent from nodes that you are not interested in? 

  • 0 in reply to TER
    Prodigy 60 points

    Uhhh, you are correct, there is xtal 40MHz. I am not using addressing because it is important that data has only 3 bytes.
    Is it possible somehow to fix this issue with different settings in registers od CC1200? 

  • 0 in reply to Ian Def
    TI__Guru**** 317180 points

    I don't believe it's possible to change this with settings since this is due to the physical implementation of the radio and also RF. Meaning that a signal will be mixed in in various degrees in the RF frontend due to phase noise and spurs.  

  • 0 in reply to TER
    Prodigy 60 points

    I understand, thanks TER. Could you recommend another CC device which has better isolation to unwanted frequencies?
    I am using it for real-time data so I am worry that if sometone is using or noising the multiple of the xtal frequency I would have a lot of bad packets.

  • 0 in reply to Ian Def
    TI__Guru**** 317180 points

    As I indicated all devices out there will have a similar problem. 

    But: Is this realistic in a real life system since either you are operating on the 868 MHz band or the 915 MHz band and these bands are less than 40 MHz wide.

  • 0 in reply to TER
    Prodigy 60 points

    I am just worried if something else eg. mobile operater is +-40Mhz from me, that can compromise my data.

  • 0 in reply to Ian Def
    TI__Guru**** 317180 points

    You will have poorer sensitivity at multiple of the xtal frequency but mobile operators etc will not send with the same sync word etc as you and hence you will not receive this traffic.

  • 0 in reply to TER
    Prodigy 60 points

    Thanks, I just want to tell you that I confirmed that carrier frquency has not to be multiple of the xtal frequency, you can choose whatever freq etc 865Mhz and you can receive correct data with receiver which carrier is +40Mhz, in this case 905Mhz.

    I think that frequencies which are mixed together are main carrier freq +- N x crystal freq, in audio it is called inter-modulation distortion.

  • 0 in reply to Ian Def
    TI__Guru**** 317180 points

    My bad. I see that I was not precise enough, when I wrote "multiple of the xtal frequency" the intention was to also include carrier freq +- N x crystal freq.