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Starting from a signal, how to determine correct Samples/Symbol value?

Here a link to the file i'm using: gfile271

And there is my flowgraph: enter image description here

fraschizzato
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1 Answers1

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By observation!

First, make sure you've correctly frequency-synchronized, i.e. your signal is centric to f=0. While you're doing that, filtering the signal to contain mostly payload signal and least feasible noise is probably a good idea.

Then, find the periodicities in your signal: That should be your symbol rate. Use a resampler (MMSE resampler, for example, was called "fractional resampler" until recently) to make that symbol rate fit into an integer amount of samples - tada, that's your samples per symbol now!

Marcus Müller
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  • Using "QT time sync" i see that signal is centered. For noise i've tried with and without low pass filter (sample_rate/2) but can't see appreciable difference in time sync, however output file changes a lot. Same for fractional resampler (Const source and resample ratio at 1) when changing phase shift. With "rational resampler" (all var set to 1) it fits, but wich is gfsk-demod samp/sym? – fraschizzato Nov 20 '17 at 11:27
  • You can't check frequency centering with a time sink. You need a frequency sink. – Marcus Müller Nov 21 '17 at 13:31
  • ok.. Now i'll make some try... I know that my start data (grabbed with rtl_433) is not on center frequency... Thanks – fraschizzato Nov 21 '17 at 16:13
  • Center frequency is obatined by using "Frequency Xlating FIR Filter", in Center Frequency the value is the offset (difference between FFT sink center and signal center). For Samples/symbols the way is counting symbols (1,0) in a knowed time then divide that count for the time (in seconds), and then divide the sample_rate for the previous value. (I've used audacity and quadrature demod with gain 1 to take a try)... Now i'm trying to find the correct value for fsk_deviation_hz... – fraschizzato Nov 22 '17 at 02:53