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CC1125: About Adjacent Channel Selectivity

xujj
Prodigy 150 points
Part Number: CC1125

dear:

In CC1125 document , it describes "Adjacent Channel Selectivity: 67 dB at 6.25-kHz Offset ".
But we can only reach 58dB at 50kHz offset. Our CC1125 config is wireless:4800bps ,2GFSK, XTAL:40M Hz,RF Freq:868.5M,Synchronous Serial Mode,Dev 0,single carrier.How to improve it?
On the other hand,the Adjacent Channel Selectivity is good at 450M in our product,it can reach 62dB at 50kHz offset.

  • 0
    TI__Mastermind 27145 points

    Hi,

    As per the data sheet (Table 4.10.3),  in 868MHz band, Adjacent Channel selectivity at 6.25KHz offset is 62dB. This spec is valid for 0.3kbps, 2-FSK, 1KHz Deviation, 3.8KHz Rx Filter BW with 40MHz TCXO.  

    According to your post, you are using 4.8kbps (no mention of deviation and Rx Filter Bandwidth). So, the above spec is not valid.

    When we change the Rx Filter Bandwidth then Selectivity and Blocking changes. Please refer to data sheet, table 4.10.3 for Selectivity spec at different data rates and at different Rx filter Bandwidths.

    It seems your measurement looks OK, if you have used different Rx Filter BW than 3.8KHz.

    Thanks,

    PM

  • 0 in reply to PM
    Prodigy 150 points
    Hi:
    As you suggested, I configure the register to those in table 4.10.3,SymbolRate:0.3kbps,Rx filter Bandwidth:3.8kHz,Deviation:1kHz,Modulation:2-FSK,XTAL:40M Hz,RF Freq:868.5M,Synchronous Serial Mode.
    But we can also only reach 58.5dB at 50kHz offset.For the previous configuration, we can only reach 56dB.It improve 2.5dB,but not reach our goal for 60dB.
    Are you have any suggestion?

    Thanks.
  • 0 in reply to xujj
    TI__Mastermind 27145 points
    Hi,

    How did you measure the Selectivity?

    Desired signal should be 3dB Higher than the Sensitivity level for this test.

    Are you using TCXO?

    If there is an offset in the frequency then you may not achieve the same spec.

    Did you test in the RF Shield room?

    If there are any interferes in the test environment then it will affect the measurement.

    Did you test it on our reference design (just for comparison purpose?


    Thanks,
    PM
  • 0 in reply to PM
    Prodigy 150 points

    Hi:

    Thank you for your reply.

    We have an external PA,our test power is 20-30dBm, .
    We use the TXCO,have no frequency offset.
    We test in indoor environment, not in the RF Shield room.
    We have tested the 450M band and the performance is ok.

    The figure below is our test results.
    Look forward to your reply,thank you.
  • 0 in reply to xujj
    TI__Mastermind 27145 points
    Hi,

    It seems you are measuring Transmit Power in the Adjacent Channel and alternate channel. This is not a Receive Selectivity test. I thought you refer to Receive Selectivity spec.

    What is your spec? What Standard you are using?

    This spec depends upon the Data rate, Freq Deviation and Phase Noise.
    In this case , it seems it is limited by the Phase Noise.
    As per the data sheet (Table 4.12.1), the Phase Noise at 10KHz Offset is -101dBc/Hz and at 100KHz Offset is -102dBm/Hz.
    If you integrate this into 20KHz Bandwidth then you will get -101+43 = -58dBm in 20KHz BW. So, your measurement meets the data sheet spec.

    Where as at 433MHz, the phase Nosie at 10KHz offset is -107dBc/Hz. If you integrate this into 20 KHz BW then you will get -107+43 = -64dBm/20KHz.

    That's why it passes in 433MHz band and not passing in 868MHz band. This is a Phase Noise limitation in the chip.

    Thanks,
    PM
  • 0 in reply to PM
    Prodigy 150 points

    dear PM:

    Yes,we are measuring Adjacent and alternate channel power.I am sorry that I made a mistake in previous.
    We use the standard "ETSI EN 300 113 V2.2.1 (2016-12)",It provides:"For a channel separation of 12,5 kHz, 20 kHz and 25 kHz, the adjacent channel power and the alternate channel power
    shall not exceed a value of 60,0 dB below the transmitter power (conducted) without the need to be below 0,2 μW"
    As you describe"If you integrate this into 20KHz Bandwidth then you will get -101+43 = -58dBm in 20KHz BW",If we integrate this into 12.5KHz we can improve the index.
    But how can we compress the signal width? We try to modify the Data rate, Freq Deviation,it,s invalid.

    Thank you!

  • 0 in reply to xujj
    TI__Mastermind 27145 points

    Hi,

    Yes, I went through the EN 300 113-1 Std and found that your Measurement method is wrong.

    You should use the Integrated BW to 8.5KHz (for 12.5KHz Channel spacing) not 20KHz. You have used 20KHz Integrated BW on the Spectrum Analyzer in your measurement. Please change it to 8.5KHz.

    If you use 8.5KHz IBW, you will get -101+39.3 = 61.7dBc then it will pass the Spec of 60dBc.  

    Also please make sure you use the following Spectrum Analyzer Settings. This is also from EN 300113.

    Thanks,

    PM

  • 0 in reply to PM
    Prodigy 150 points

    dear PM:

    Thank you for your reply.

    According to the new test method, we have tested again. Our test results are as follows:
    use 8.5KHz IBW we can get 61.5dBc
    use 16KHz IBW we can get 59.5dBc
    The result can meet the chip spec,But the 16KHz IBW can,t meet the standard requirements. 

    Thanks.

  • 0 in reply to xujj
    TI__Mastermind 27145 points
    Hi,

    You can declare your equipment for 12.5KHz Channel spacing then they will test at 8.5KHz IBW. So, it will pass the Spec.
    Please discuss with your Certification Lab about the measurement and spec limits.
    This is a Limitation on the chip.

    Thanks,
    PM
  • 0 in reply to PM
    Prodigy 150 points

    dear PM:

       OK, Thank you very much!