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CC1125: Minimum receiver bandwidth

Milos Milosavljevic
Prodigy 20 points
Part Number: CC1125
Other Parts Discussed in Thread: CC1200

Dear Support Team,

I have a question about the CC1125 RX bandwidth. According to the data sheet the RX bandwidth has to be minimum twice the data rate.

However, I was wondering if that can be lowered? I forced that change in the CC1125 registers (ie dropped it to 17.9 and 16.7kHz) but had no luck to demodulate a signal. So I dont know if there is something happening at the background. I do receive it when it is twice the data rate or higher. My data rate is 9600bps and my signal is about 10kHz wide including the first sidebands. So there is really no need to have the 19.2kHz of RX bandwidth (ie. 2 x 9600). 

Any comments are more than welcomed.

Many thanks

Milos

  • 0
    TI__Mastermind 39860 points
    The RX BW has to be >= 2 x Symbol Rate. This is given by the design.

    With 9.6 kbps data rate the signal will be wider than 10 kHz. Approximate signal BW can be calculated using Carson's rule as SBW = symbol rate + 2 x deviation. This equation is fairly accurate for 2-FSK, but for 2-GFSK it will be somewhat conservative. Assuming 2-GFSK modulation and a modulation index of 1 (= deviation of 4.8 kHz) the signal BW will be approx 15 kHz. . In addition, the frequency offset between the RX and TX units must also be taken into account. 19.2 kHz RX BW is probably an absolute minimum BW in your case. Depends on the crystal accuracy and also whether FB2PLL is enabled (see the user guide for details about the latter).
  • 0 in reply to Sverre
    Prodigy 20 points

    Thanks for coming back to me.

    The modulation is 2GFSK and freq deviation is 2.4kHz. The output signal bandwidth is about 12kHz. Please see below. Assuming no freq offset between RX and Tx surely this should be demodulated with RX bandwidth lower than 19.2kHz, right? So my question is what happens when registers in CC1125 are set to lower than twice the data rate? Does this cause the transceiver to be in a weird state or something?

  • 0 in reply to Milos Milosavljevic
    TI__Mastermind 39860 points
    Sorry, but with 9.6 kbps data rate the RX filter BW has to be 19.2 kHz or wider. This is how the radio was designed. This limitation was removed on the next generation, CC1200, but for CC1125 you will not receive anything if you set the RX filter BW to less than 19.2 kHz.