On 12/17/12 06:13, Matt Ettus wrote: > On Sun, Dec 16, 2012 at 2:27 PM, Joanna Rutkowska < > joa...@invisiblethingslab.com> wrote: > >> >> This is an FSK receiver from Atmel. Even though it apparently supports >> only data rates between 1-20 kbps, its supported Fd is within a range of >> 18kHz-50kHz. This seems to contradict "your" theory above -- why would >> Atmel use such a wide bandwith if they could just use no more than 20kHz >> for 20kbps? >> > > > Joanna, > > > Increasing deviation beyond the deviation of MSK (i.e. the minimum) does > not help performance, but it can help ease receiver design. > > The reason to use a very high deviation at a low data rate is to make it > easier to deal with frequency error with low cost crystals. If you have > low cost 50ppm crystals on both ends, you could have 100 ppm total error. > 100 ppm at 450 MHz is 45kHz. Spreading the 2 tones further apart allows > you to guarantee that one will always have positive deviation and the other > will have negative deviation. > > The FSK demodulation method with 2 filters is almost never used. This chip > likely uses either direct frequency measurement (derivative of phase) or a > PLL. > > Matt >
Thanks Matt. This makes sense. joanna.
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