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

From: Joanna Rutkowska
Subject: Re: [Discuss-gnuradio] FSK performance as a function of Fd?
Date: Mon, 17 Dec 2012 23:42:26 +0100
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On 12/17/12 06:13, Matt Ettus wrote:
> 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?
> 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.


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