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Problems with CC1110 at 915 MHz frequency

Aitor Larruskain
Prodigy 100 points

Following the directives of AN006, we are using the 177kHz deviation and FBW of 540 kHz. From our previous designs we follow a rule that you recommended us ( DN005)  that the FBW should be at least  BWchannel > BWsignal + 4· XTALppm· fRF. We are using a +/-30 ppm crystal oscillator , working at 915 MHz with a 177 KHz deviation so at  least

  BWchannel >    BWsignal(3*177KHz) +     4· XTALppm· fRF( 4*30*915)  >   640 KHz.

As you see if we follow the An006 we should use a configuration of  177kHz deviation and FBW of 540 kHz, but if we follow the DN005   the FBW should be at least 640 KHz.

We have made test with the    177kHz /540  kHz configuration and there  are problems of CRC, we suppose that it is because of the FBW.

We have also made tests with a 177kHz/700kHz configuration but we see that we have big problems of communication because we loose sensitivity in an extreme way.

Is there any way of meeting FCC 15.247  with CC1110 working properly ( good communication)? Or is it just impossible?

  • 0
    TI__Mastermind 39860 points

    What is the data rate in your application? The data rate and frequency deviation are factors that sets the signal bandwidth. Your calculations imply that you have a data rate of 177 kbps.

    BWchannel >    BWsignal(3*177KHz) +  4· XTALppm· fRF( 4*30*915)  >   640 kHz.

    There is a trade-off between crystal accuracy and sensitivity since a wider receiver bandwidth is needed when using less accurate crystals. What you could do is to test sensitivity vs frequency offset similar to DN005. This measurement will tell you what crystal accuracy is needed fora particular RX filter BW. Note also that you can "extend" the RX filter BW by enabling frequency offset compensation through register FOCCFG[1:0]. Setting these bits to 10b will move the digital filter +/-BW/4, thus increasing the RX filter BW by BW/2. The noise BW, however, will still be equal to the programmed RX filter BW.

     

     

  • 0 in reply to Sverre
    Prodigy 100 points

    The data rate of my application is 38400 bps, but if the deviation needed for fulfill the 6dB requirements of FCC 15.247 is 177kHz at least the FBW should be 3 times the deviation +  4· XTALppm· fRF( 4*30*915)

  • 0 in reply to Aitor Larruskain
    TI__Mastermind 39860 points

    As a first approximation the RX BW should be 38.4k + 2 x 177k + 4 x 30 x 915 = 502 kHz

    I do not understand where you get the factor 3 from in your calculations.