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CC1125: Minimize ACP in TX - TX filter BW in CC1125

DanielU
Prodigy 95 points
Part Number: CC1125

Hi,

I am designing a radio product with CC1125 that will be compatible with a legacy protocol in FSK. Thus I am using the Transparent Serial Mode, which works as expected. Now I want to minimize the Adjacent Channel Power (ACP) and I have realized that the User's Guide (swru295e) is very brief when it comes to TX data filtering. By playing with register values however, I note that there is an impact on the modulation filter bandwidth from the Symbol Rate that I set (even though it is not doing any synchronizing in the Tranparent Serial Mode). There is also an impact from the UPSAMPLER_P value. Are these the parameters that I should play with?

Also, I see a not-so-clean spectrum around the carrier due to spurrs at kHz-offsets. Can I play with and optimize any register values for the Frequency Synthesizer to mitigate this?

Thank you.

  • 0
    TI__Guru**** 317180 points

    - For TX it's very few reasons to use Transparent Serial Mode. If you have a legacy protocol that doesn't use a typical format with preamble + sync +.. you can disable preamble and sync and place everything you want to send on the air into the TX FIFO. 

    - Symbol rate has an effect even in transparent mode since this sets how the input data is sampled in the modem. If I remember correctly the data is sampled internally with x8 the symbol rate meaning that increasing the symbol rate will improve the resolution in TX. I have never tried TX and transparent mode and how the spectrum is affected since, as stated in the last bullet, the FIFO can and should be used.

    This appnote: https://www.ti.com/lit/pdf/swra424 contains some information on how the synth settings can be changed. 

  • 0 in reply to TER
    Prodigy 95 points

    Thank you for quick reply!

    We have legacy code for generating the messages so Transparent Serial Mode is actually very handy also for TX. But, I have now realized another big advantage with this: I can set the modulation format to GFSK and set a much higher symbol rate than what I have. This gives me a much better filtering of the transitions than in FSK mode, while I still can have flat tops and bottoms on the data bits to be compatible with the FSK-receivers of the legacy products in the other end. Excellent, actually! Looks great in the spectrum.

    Would I need to set it back to FSK, and turn down the symbol rate, for the receiver? Do it care about the shape of the frequency shift in Transparent Serial Mode? Guess it does not, since it is hard limited. Or is the RX filter influenced?

    And setting "FS_CHP register to 0x3F after calibrating the PLL" as pointed out many times in the appnote (and also once in the User's Manual), in the context of receiver performance, actually helps mitigating the spurrs in TX. Great!

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

    In receive you will have some jitter meaning that the output should be oversampled by the MCU to suppress this. Having a high symbol rate in RX will decrease the jitter.

    FSK vs GFSK should not matter since the output signal is taken early in the RX chain before any special filtering. 

  • 0 in reply to TER
    Prodigy 95 points

    Sorry, are you talking about what I set as Symbol Rate in the registers or the actual symbol rate that I have on air? And the jitter should in general be reduced by lowering the symbol rate, right?

    Question rephrased,
    Is the value written to SYMBOL_RATE influencing anything in RX in Transparent Serial Mode?

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

    Sorry for being unclear, I was referring to the register setting. Meaning 

    DanielU said:
    Is the value written to SYMBOL_RATE influencing anything in RX in Transparent Serial Mode?

    Yes