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CC1125 4FSK root cosine filter

Other Parts Discussed in Thread: CC1125

Hi,

Is there root cosine filter in 4FSK mode of CC1125?

We have found that the sideband rejection was not good.

  • Johnson Zhang1,

    No, the CC112x series products support Gaussian (BT=0.5) filtering only.

    Can you be more specific about your findings?

    Regards,
    /TA

  • Hi,TA12012

    Thanks for your reply.

    We are using CC1125 to design a new radio modem, but our old product has used a baseband IC to acomplish encoding and decoding.  And the baseband IC has root raised cosine filter in both transmit and receive modes.

    We have compared the spectrum with modulation, and found that the sideband rejection is different seen as following pictures.

    Spectrum of baseband IC:

    Spectrum of CC112X:

  • And we found that the receiver perfomance was not good when communicated with old product which used baseband IC.

    Transmitter                                                              Receiver                                                    Result

    CC1125                                                                   Old product                                               OK

    CC1125                                                                   CC1125                                                     OK

    Old product                                                              CC1125                                                     No good, there was error bits

  • The BT value of Gaussian fiter is variable or fixed?

    Can we change the BT value by user register or reserved register?

  • The BT is fixed.

    Since the chip does not support root cosine directly you can look into the "Custom Frequency Modulation" feature (section 5.2.4 in the UserGuide) and use the raw data to code/ decode.