Re: [Discuss-gnuradio] GMSK: GSM initial phase for a burst

[Matt Ettus]

Jeffrey Karrels wrote:
> Hello everyone.
>
> I am looking at the frequency correction burst of a GSM signal.
> According to spec the original data defines 148 bits of 0 that are
> differentially encoded and GMSK modulated.  After the data goes
> through the differential encoder it ends up as [-1,1,1, (142 1s),
> 1,1,1].  I have taken this burst and plotted I vs Q (which is
> attached).  I was expecting to see the amplitude ramp up followed by a
> negative pi/2 phase shift, followed by 147 positive pi/2 phase shifts,
> and a ramp down. I never see the initial negative phase shift though.
> Am I missing something in my sanity? Any thoughts or ideas?

Its hard to read the graph like that.  Plot angle vs time or even
better, derivative of angle vs. time.

Matt