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CC1125 CFM/Analog FM

He He
Prodigy 110 points
Other Parts Discussed in Thread: CC1125

Hi experts:

     I want to use CC1125 as RF transceiver for our digital PMR device(BW: 12.5KHz, 4FSK, 9.6Kbps )!

     According to user guide of CC1125, I have to use CFM/Analog FM mode format with transparent mode, from user guide 5.2.4 as following:

     The signal CFM_TX_DATA_CLK can be output on a GPIO by setting IOCFGx.GPIOx_CFG = 30. This

signal will be asserted every time the CFM_TX_DATA_IN register should be written and should be
used as an interrupt to the MCU to synchronize the SPI data to the internal modulation rate. The
signal runs at 16x the programmed symbol rate.
      The signal CLKEN_CFM should be output on a GPIO (IOCFGx.GPIOx_CFG = 29) and used as a
trigger to read the CFM_RX_DATA_OUT samples. This signal runs at the same rate as the
programmed symbol rate.

      Looks in Tx status, the data rate from MCU to CC1125 is 16*symbol rata(4.8K), but in Rx status, the data rate from CC1125 to MCU is just 1*symbol rate?

      Becasue in Rx state, MCU have to do demodulate by itself, so just 1*symbol rate, I think it doesn't work for my case!

      Could you give me some suggestion for my case?

br

Haibo

     

  • 0
    TI__Guru**** 317180 points

    Have you looked at the examples here: http://e2e.ti.com/support/wireless_connectivity/f/155/t/165695.aspx

    It should be possible to set a higher symbol rate on the receiver to force oversampling.

  • 0 in reply to TER
    Prodigy 110 points

    Hi TER,

        Thanks for your information!

         I had review this post you mentioned, but I don't understand it's possible to set diffirent symbol rate for Tx or Rx?

    br

    Haibo

     

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

    If the chip do de-modulation the de-modulator needs to know the symbol rate and the rate has to be the same both on the Tx and Rx side. But when you using analog FM the raw data is read directly from a register and the symbol rate is not used directly in the receiver. When increasing the symbolrate the RX BW has  to be increased too. (RX BW > 2*symbol rate.

  • 0 in reply to TER
    Prodigy 110 points

    Hi, TER

        Thanks for your comments!

        As your mentioned, looks I can't use CC1125 any more? because for my case, receiver BW must be fixed at 12.5KHz, and symbol rate must be 4.8K, so how can I get oversampling raw data?

        Br

    Haibo

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

    Could you elaborate some why you need oversampling on the received samples?

  • 0 in reply to TER
    Prodigy 110 points

    Hi TER,

        For my case, Baseband  signal is 4FSK modulation, and symbol rate is 4.8K, so basically double times sampling rate of symbol rate is needed for baseband demodulation, and to good performance, generally we use 8 or 16 times oversampling!

        And it's same reason, in Tx, CC1125 needs 16 times symbol rate!

        Anyway, I want to know how can I get a good analog baseband signal for CC1125?

        Thanks!

    Br

    Haibo

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

    If you are using 4FSK, 4.8ksps, why do you want to use analog FM and not the built-in packet engine? (or synchronous mode if you need the data in a serial stream)

    Note that for analog FM the output is not analog but the deviation of the signal. For 4FSK the deviation has only one value pr symbol and hence no need to oversample.

  • 0 in reply to TER
    Prodigy 110 points

    Hi TER,

        Yes, Surely I want to use built-in packet engine, but CC1125 can't provide proper sync mode for my case, so I have to use analog FM mode!

         Yes, for 4FSK, one symbol for one fixed deviation, but for baseband demodulation, basically I have to oversampling to sync symbol first and then can demodulate symbol to bits!

    Br

    Haibo

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

    What type of sync mode do you need? You can even run the radio without sync word and look for it in software afterwards (in this case you should use carrier sense)

    Could you elaborate a bit on the last statement? Since you have the deviation why do you have to oversample?

  • 0 in reply to TER
    Prodigy 110 points

    Hi TER,

         The following is frame format, for your reference. so do you think CC1125 can support this sync mode?

        

             If without sync, how CC1125 to do demodulation?

             About oversample, for my case, one symbol just one deviation, How can I do symbol sync? before I can do demodulation, I have to do frame sync and symbol sync firstly, so oversampling is necessary!

            Thanks!

    Br

    Haibo

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

    Just to clarify : Is it no preamble or sync before the first 108 bits of payload? How do you know you receiving payload and not noise? (regardless of the method you are using to receive the data)

  • 0 in reply to TER
    Prodigy 110 points

    Hi TER,

         No, there are no preamble or sync before the first payload. 

         So, First, our MCU received the oversampling raw data and buffer one frame, then do sync and demodulation, and  then high layer process!

        Br

    Haibo

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

    Is the same payload sent twice? CC112x needs to receive a few bits to settle the AGC before receiving data.

  • 0 in reply to TER
    Prodigy 110 points

    HI TER, 

         No, it's different payload, but every communication starting, some same frames will be send on air.

         Also AGC is a big problem, because in my case it's time division system, there are two time slot, one time slot is 30ms, so I want to know how long receiver need to settle AGC? 

    Br

    Haibo