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CC110L: Questions about setting "Rx filter BW"

Jianyi Bao
Expert 2220 points
Other Parts Discussed in Thread: CC110L

I want to use RF studio to configure my CC110L for: 1. 433M 2. 250kbps 3. GFSK

In RF studio, "Rx filter BW" is set to 541.6kHz. According to swra122c, BWchannel > BWsignal+4*XTALppm*fRf. Here are my questions:

1. when RF studio calculates "Rx filter BW", what crystal error it is using?

2. There seems no place for user to specify their own crystal error.

3. How can we get the "BWsignal" using a spectrum analyser for GFSK? 99% of the power?

Thanks

  • 0
    TI__Mastermind 39860 points

    SmartRF Studio does not calculate the RX filter BW. The user needs to enter the BW and make sure it is wide enough to fit the signal and account for any frequency offset between TX and RX units.Carson's rule can be used to estimate the signal BW as data rate + frequency separation (for 2-GFSK). This is a conservative rule so you can use a spectrum analyzer to measure 99% occupied BW. There are also mathematical models that give good estimates.

    Used an internal calculator and got 99% BW of 431 kHz for 250 kbps, +/-127 kHz deviation, 2-GFSK.

  • 0 in reply to Sverre
    Expert 2220 points

    Hi, thank you for your feedback.

    Here are some questions:

    1. What is the "internal calculator" or "mathematical models" that could be used to calculate the bandwidth?

    2. I was told that the parameters in RF studio UI has been optimal, and should not be changed easily. So according to you, this is not the case, at least for "Rx Filter BW". I believe "Channel spacing" is also not. So what about "Deviation"? How to set this value for my own system? I was told that the smaller "deviation", the worse sensitivity.

    Another question is that: Let's say now "Rx filter BW" is 500k, so can I set "channel spacing" also to 500k, and then in my system use channel 0 and 1 (continuous channels) and the channels will not have interference with each other? Because 99% of the power is within the channel, and only 1% of the power from channel 0 will enter channel 1, and I think the 1% interference should not be big problem.

  • 0 in reply to Jianyi Bao
    TI__Mastermind 39860 points

    1. We use a Matlab script to calculate the 99% OBW. You can use a spectrum analyzer to measure the 99% OBW or use Carson's rule as a first order approximation.

    2. The settings in Studio is a good compromise between data rate, deviation, RX filter BW and crystal tolerance. Theoretically, there is an optimum deviation/datarate setting if you simultaneously minimize the RX filter BW. Every halving of RX filter BW improves sensitivity with 3 dB whereas sensitivity vs deviation/datarate degrades with about 1.5-2.5 dB per halving down to a certain limit where the loss increases very fast.

    A good compromise is usually to have a modulation index of 1 (i.e 2*deviation/data rate = 1).

    Last question: You are correct about the channel spacing and the interference. With RX filter BW of 500 kHz you can use a 500 kHz channel spacing. That said, wider spacing (e.g use channel 0, 2, 4, 6....) will giveless interference (i.e. better selectivity)

  • 0 in reply to Sverre
    Expert 2220 points

    Thank you so much. so maybe dev=127k for 250kbps is a good value.