Re: [Discuss-gnuradio] Ettus N210 GMSK 9600

[Andy Walls]

>                              From: 
> Tom Golden
>                           Subject: 
> Re: [Discuss-gnuradio] Ettus N210
> GMSK 9600
>                              Date: 
> Sat, 26 Mar 2016 10:12:18 -0600
> 
> ______________________________________________________________________
> I've been staring at the de-modulator and I really think it's correct.
> Unfortunately amateur packet radio isn't well documented (at least for
> me).
> 
> 
> I think there's a scambler I'm not taking into account.  Does anyone
> know the specifics of this?  Or can point me to how to what the
> parameters of the descrambler block mean and/or how to determine their
> settings (my best guess is it's a 17-bit LFSR with a polynomial of 1
> +x^12+x^17)?


I can't help with the descrambler, sorry.  There are some AX.25 pages
out there that talk about it; you've probably seen them already.

Looking at what the radio outputs when the Tx power ramps up, it's a
good bit of unmodulated noise, and then some varying periods of the high
freq, followed by the low freq, followed by a clock pattten, followed by
what looks like an HDLC flag,  but that's where it stops making sense.

There are about three instances of these in the data file.  I've
attached a window shot of one of them.  (Observe the green line, the
transmitter ramps up rapidly at about the 14.5 ms mark on the top plot).
All the close-by data bits appear to be the same on every instance of
this start of burst.

So a preamble to look for in the NRZI bits is: [0]+[0]*12+[0,1]*4+[0]*4


I imagine the following is deliberately done by the transmitter, for the
sake of the receiver:

1. Initial high tone and low tone for AGC and for AFC adjustment
2. Clock pattern for clock sync.
3. A long-ish initial low followed by a short clock pattern, so the
scrambler doesn't muck with the clock pattern.  (If a scrambler is even
at issue here.)

Regards,
Andy

Screenshot from 2016-03-26 14:50:58.png
Description: PNG image