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Re: [Discuss-gnuradio] FSK performance as a function of Fd?

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] FSK performance as a function of Fd?
Date: Sun, 16 Dec 2012 21:13:47 -0800

On Sun, Dec 16, 2012 at 2:27 PM, Joanna Rutkowska <address@hidden> 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?


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.


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