• State
  • Locked Locked
  • Replies 4 replies
  • Subscribers 17 subscribers
  • Views 2547 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

CC1101 - SmartRF Studio and frequency calculations

Stefano Infante
Expert 2310 points
Other Parts Discussed in Thread: CC1101

Hello.

In SmartRF Studio for CC1101 at 433 MHz, if I select, among the preferred settings, 100 kBaud sensitivity optimized, I obtain:

  • Datarate about 100 kBaud
  • Frequency deviation 47.6 kHz
  • RX filter bandwidth 325 kHz.

According to Carson's rule, the signal bandwidth is about 2(Delta-f + fm), where Delta-f is the frequency deviation and fm is the maximum frequency of the signal. This gives 2(47.6 + 100) = 295.2 kHz.

The CC1101 datasheet (page 35, chapter 13) states that "For best performance, the channel filter bandwidth should be selected so that the signal bandwidth occupies at most 80% of the channel filter bandwidth".

80% of 325 kHz is 260 kHz, so it appears that the bandwidth of the signal is wider than 80% of the channel filter bandwidth.

Moreover, if I select, among the preferred settings, 250 kBaud sensitivity optimized, I obtain:

  • Datarate about 250 kBaud
  • Frequency deviation about 127 kHz
  • RX filter bandwidth 541.67 kHz.

Again, according to Carson's rule, the signal bandwidth is 2(250 + 127) = 754 kHz, far greater than 541.67 kHz!

What's wrong with that? What am I missing? (Sorry, but I'm not an RF expert...)

Another problem I am not sure to understand is about channel spacing: SmartRF Studio appears to leave channel spacing at 200 kHz, disregarding all other settings. But if the receiver filter bandwidth is set to 541 kHz, does it mean it takes almost 3 channels? So, in a multichannel environment, I should only select, say, channels 0, 3, 6, and so on, should I?

The last question: when I set the RF frequency, am I setting the center frequency or the base one? In other words, if I set, say, 432.9 MHz with a receiver filter bandwidth of 200 kHz, will the bandwidth of the receiving filter go from 432.8 to 433.0 MHz or from 432.9 to 433.1 MHz?

Thanks,

Stefano

  • 0
    Expert 2310 points

    Hi, can anyone help about this topic?

    Thanks

    Stefano

  • 0
    Prodigy 110 points

    Stefano,

    I too anticipate an answer to your questions, thought they were great and likely common concerns for others. 

    I had similar but the reverse question.  For example if I said that I did not want the signal bandwidth to be greater than 100 kHz, how would I choose data rate, deviation, and RX filter in RFStudio to limit the design to the specified bandwitdth while still achieving as high and reliable data rate as possible. 

    Hopefully someone will comment,

    mlk

  • 0
    Genius 3020 points

    I too think these are good questions.  We currently are trying to figure out the Deviation to RX filterbandwidth relationship. 

    Any help would be greatly appreciated,

    Thanks,

    BB

  • 0
    TI__Mastermind 18780 points

    Hi,

    I understand that this can be confusing, and to deside the optimal RX filter bandwidth is not a staight forward process. Carson's rule is good for guidance, but depending on application, choice of crystal etc, there will be a trade-off between RX filter bandwidth versus sensitivity that must be considered. The deviation specify the nominal frequency deviation from the carrier frequency for a '0' (-DEVIATION) and a '1' (+DEVIATION). The optimal deviation for different datarates will depend on application. Deviation affects choice of RX filter bandwidth and thus also sensitvity, and in TX it affects modulation bandwidth. In TX, a smaller deviation means less occupied bandwidth. Higher data rates requires higher deviation though. If your application have a fixed RX filter bandwidth, you should maximize the deviation that will fit within that bandwidth (leaving some room for frequency deviation due to crystal tolerances) to get the best sensitivity. For applications that allow changing the RX filter bandwidth, there is theoretically 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. When we optimized CCxxxx settings, we selected the frequency deviation that gives the
    best sensitivity but also allows for a certain crystal frequency offset. Also we looked at optimizing the ADC performance for the specified setting to avoid that noise from the ADC decreasing the sensitivity. The IF frequency is thus also very imporant to choice correctly as it positions the receiver bandwidth within the ADC frequency response. Optimum IF frequency has been selected for each preferred setting (RX filter BW) in SmartRF Studio to ensure best possible performance and robustness.  We always recommend to, if possible use the preffered settings from SmartRF Studio, at least use them as a starting point before starting to alter parameters. The settings are well tested over conditions and based on our experience, they are the easiest way to achieve the best performance. See also design note DN005 that explaines for the CC11xx products how the sensitivity changes with frequency accuracy, frequency offset, filter bandwidth etc.

     

    Charlie