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Choosing modulation method

Lance Doyle19477
Prodigy 10 points

Other Parts Discussed in Thread: CC2500

The CC2500 is capable of implementing four different modulation methods.

Given that the hardware is already in place and there is no cost difference in choosing one method over another, why would I choose one method over another?

Which method is going to yield the best transmission/reception characteristics?

Which method is most likely to be easier to pass the FCC test?

We are working on a new product and have the chance to make the "right" decision.

  • 0
    Mastermind 8170 points

    First per Ti/Chipcon OOK was included for compatibility with older systems.  Since it is a simple On - Off strategy it is easy to implement and was the choice for very low cost consumer items (garage door openers, TV remotes) before the current sophisticated RF and IR devices were available. It is the least robust of the waveforms.

    This is VERY cursory and there are many factors not considered.

    FSK, 2FSK, and MSK are variations on the same theme. They all deviate between 2 frequencies.  At a top level the primary difference is the amount of frequency deviation versus the rate of deviation.

    A larger difference in the two frequencies F2- F1 for a given modulation rate gives promise of being able to distinguish the one state from the other more easily which translates into “with less transmitted power or more range” but the trade is an increase in required bandwidth which brings with it more noise (requiring more RF power) and less efficient use of spectrum. At the lower modulation rates the gain of greater F1 – F2 separation more than offsets the disadvantage of the required additional bandwidth and the bandwidth is intrinsically low at these rate to start with. As an example with a modulation rate of 1KHz a transceiver with a F2-F1 of only 1KHz spacing at 2.4GHz would be at a disadvantage due to “close in” noise from components such as oscillators used in the transceiver. For MSK  is F2-F1/modulation rate = .5 which happens to correspond to the best spectrum efficiency (i.e. bits per bandwidth) for FSK.  MSK is also unique relative to other forms of FSK in that is does not have discrete frequency components at F1 and F2 and actually falls off faster than QPSK.

    So back to the question: I would consider OOK off the table.  As the data rate increases you will find an advantage with FSK, then 2-FSK and finally MSK. Similarly you will find FCC compliance becomes more difficult as the data rate increases. Here are a couple of drivers; you are spread over more spectrum, and with this increased bandwidth is increased noise driving you to higher output power. The regs are written to define a box, higher data rates push you to challenge the edges of box. So at low modulation rates FSK, and  2-FSK have the advantage. At 500Kb/s you will be using MSK.

    Hope this helps

     

    -H