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

From: Michael Ossmann
Subject: Re: [Discuss-gnuradio] FSK performance as a function of Fd?
Date: Sun, 16 Dec 2012 19:39:24 -0700
User-agent: Mutt/1.5.21 (2010-09-15)

On Sun, Dec 16, 2012 at 11:27:47PM +0100, Joanna Rutkowska wrote:
> Thanks for the detailed answer! I did some more study and my
> understating is that what you wrote above applies to (theoretical)
> coherent demodulation. AFAIU nearly all practical receivers use
> non-coherent demodulation (because it's cheaper, and apparently, has
> about 1dB penalty only on sensitivity?). This non-coherent detection
> is apparently realized using two bandpass filters centered around the
> two frequencies.

I believe you are correct that sensitivity vs. deviation is strongly
dependent on the receiver implementation.  You may not be correct
assuming anything about the particular receiver implementation in the
Atmel part.

> So, my understanding goes, in any practical realization, it should be
> better (not worse at least) to use as large Fd as one can, because it
> makes the job of the two filters easier...
> But, it's apparently not like this. Here's the link to the Atmel paper
> I mentioned before (again, please take a look at Figure 9-2):
> www.atmel.com/Images/doc9174.pdf

Making a guess about the receiver implementation, I'd say you correctly
describe why the sensitivity improves as Fd increases toward 50 kHz.
I'd further guess that the reason sensitivity gets worse above 50 kHz
deviation is that the bandwidth of the signal exceeds the fixed IF
filter in the receiver.

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