Re: [Discuss-gnuradio] Decoding 2FSK Compensating for carrier jitter/skewing (CFO)

[Andy Walls]

On Sat, 2017-07-08 at 21:38 -0400, Andy Walls wrote:
> > Date: Fri, 7 Jul 2017 19:50:55 +0300
> > From: HLL
> > Hi all,
> > 
> > I'm relatively new to DSP and gnuradio but I tried tons of stuff
> > and
> > I couldn't decode a fairly simple FSK data.
> > baudrate seems around 600-700 bps and fsk deviation is less then
> > 3k.
> 
> 
> Hmmm.  I took a look at your signal and tried building a coherent 2-
> FSK 
> demodulator.  Under the assumption that it was straight 2-FSK, the
> signaling tones looked to be at +/- 1200 Hz when properly centered.
> The
> fastest bit rate appeared to be 1880 bits/sec.
> 
> But in reality that doesn't work.  I could never get good symbol
> timing
> recovery as the "FSK" signal appeared to have two different baud
> rates.
> 
> After some reflection, the signal you have appears to actually be
> AFSK
> inside of FM.  Zoom out a little on the output of the quad demod, and
> your eye can see the two tones.  The two tones appear to be at 350 Hz
> and 940 Hz.  The tones are unusual in that they are square wave tones
> vs. sine wave.
> 
> I haven't worked you the baud rate yet.  I'll hack away at it more
> tomorrow. 

It definitely is AFSK in an FM transmission.

The baseband baud rate appears to be a 233 bits/second.

The beginning of every packet starts out with the following bits:

000000000000000000000000 100 01111110

I keyed off of those last 8 bits above (01111110), since they looked
like an HDLC flag character to me.

The payload bytes of the 4 packets in the file (after that 01111110 bit
pattern) are:

6884485b066e7505647e875a4ac70c74447474477e47e5f85c47065f44be79a74e67a6a452bfffffffff
68c4485b062c7565e67227564ac74e74247478477447e5f85ce7065f44be75a74867a6ec52dfffffffff
68c4485a278e6f856e7006706a470c7fa4f67c057c47e7f06f47447f66ae7d867048874243dfffffffff
68c4485a278e6f85ec70a67a6267cc7fa4f678257647e7726fe7447f662c7d8670488700437fffffffff

The ff's at the end are the space after the end of the packet.


Try the attached GRC file.

For it to work, you will need to build and install:

1. A relatively recent GNURadio

2. The gr-nwr OOT module found here:
https://github.com/awalls-cx18/gr-nwr
You'll have to write your own PyBOMBS recipe file, if using PyBOMBS to
build GNURadio and OOT modules.

3. The gr-reveng OOT module found here:
https://github.com/tkuester/gr-reveng

Regards,
Andy

meter_reader_afsk.grc
Description: application/xml