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CC1110 Migrating from 868 MHz to 915 MHz problems with RF setting in Smart RF studio

Aitor Larruskain
Prodigy 100 points

Hello,

We have a design that is correctly running at 868MHZ with a GFSK modulation and a  baud rate of 38400 bps. Now we have to migrate to 915 MHz. To be FCC 15.247 compliant  we have done some measurements as DN006 reccomend . From his study we got that the correct parameter to be FCC 15.247 compliant are:

Modulation ; GFSK

Manchester code enabled

Baudrate: 38400 bps

Deviation : 140 kHZ

TX power: 7 dBm

As the deviation needed to be compliant is 140 kHz the minimun  RX filter BW should be  (140*4)/0.8 = 700 KHz .  Using  smart RF studio we get that the minimun  RX filter BW  should be 812 KHz.

We use Smart rf studio with two CC1110 EM , we transmit 1000 packets  of  128 Bytes from one  EM to another and with this configuration we get a PER of  20 %. With same EM  and old parameters (868 MHZ parameters) , PER of <1%. Can you tell me where do I have the problem? are the parameters I use correct or is there something wrong.

Best regards

 

 

  • 0
    TI__Mastermind 39860 points

    To meet the 6 dB requirement of minimum 500 kHz a +/-140 kHz deviation seems to be on the low side. In our testing we used +/-177 kHz deviation

    Please find register settings attached.  You can import this file into SmartRF Studio 7.

    The RX filter BW should be 540 kHz when using 177 kHz deviation and 38.4 kBaud data rate (see table 3 in DN006). 

    Not sure about your old 868 MHz settings, but I guess you used a lower RX filter BW. If you change the RX filter BW by a factor X, there will be a theoretical degradation in sensitivity of 10*log(X). Note: this is if we keep the data rate and frequency deviation (modulation index) the same.

    When you increase the frequency deviation to meet the 6 dB BW requirement under FCC 15.247 you also need a wider RX filter BW. Theoretically, there is an optimum deviation/datarate setting if you simultaneously minimize the receiver filter bandwidth. Every halving of receiver filter bandwidth increases sensitivity with 3 dB whereas sensitivity vs deviation/datarate decreases with about 1.5-2.5 dB per halving down to a certain limit where the loss increases very fast.

  • 0 in reply to Sverre
    Prodigy 100 points

    We have tried the parameters from An006 and  we are getting some problems. When we designed the 868 MHz product we had some problems about the relation between the crystal oscillator and RF parameters. After asking you about the problems, a rule we had to follow was that the signal bandwidth did not have to be  more than 80 % of the RX filter bandwith. If we applied this rule to the 915 MHz case , as frequency deviation is 177 KHz, the total signal bandwidth is 4*177KHz = 708 KHz so the minimum RX filter bandwith should be at least 708/0.8 = 885 KHz which is not possible in Smart RF studio. Are my calculations OK or am I doing something wrong?