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

[Cinaed Simson]

Would you compare the sha512sum of your raw data file with mine?

8cbb5adfc64b2c31c3b77e3ff3ba3f1867b8c9a87712e649bdae2b2c991c2bb0088cd888f37288e1e5eb5c9c0bb96ac39a484237a06523631c133c621f546322

-- Cinaed


On 07/12/2017 04:50 AM, HLL wrote:
> Images didn't go through, here they are:
> Inline image 1
> 
> Graph:
> Inline image 2
> 
> On Wed, Jul 12, 2017 at 1:56 PM, HLL <address@hidden
> <mailto:address@hidden>> wrote:
> 
> 
> 
>     On Wed, Jul 12, 2017 at 1:56 AM, Cinaed Simson
>     <address@hidden <mailto:address@hidden>> wrote:
> 
>         It seems odd that a device with a maximum power 116 dBm would
>         have such
>         a weak signal.
> 
>     As I mentioned earlier, I'm not really sure what is the frequency of
>     the device is, I just scanned the 300 + area and I found a few that
>     correlate with the device transmission
>     This one was taken on 440.15M; I Also have another similar capture,
>     captured at 419.562M
> 
> 
>         Maybe the antenna was to close? The distance of the receiving
>         antenna
>         was roughly 0.1 of the carrier wavelength from transmitting antenna.
> 
>     I guess it was, distance was around 8-15 cm or so, as the wavelength
>     is 68.15cm, When capturing I didn't considered it (or even known it
>     is a factor)
>     The antenna used is the stock RTL SDR one.
> 
>     If you can recommend some hackish (DIY or even retail) antenna to
>     better receive that signal that would be grate.
> 
> 
>         Also, the documents indicate the channel width is from 7 kHz to
>         16 kHz -
>         the sampling rate of 8 kHz may have been to small.
> 
>     Well, As  Andy mentioned, it's not FSK, so the documentation may be
>     talking about other version of the device.
>     I Downsampled the capture to 8khz after centering because that was
>     much more then 2 times of bandwidth I've seen on the FFT on active burst
> 
> 
>         But in any case, there's a lot of good information in this
>         thread and
>         it's going to take me a while to digest all of it.
> 
>     If that helps, I kindda made a quick graph that does similar thing
>     to Andy's graph, and I got similar results:
> 
> 
>     now it seems that a large CFO glitch translates to short glitchs in
>     the digital signal
> 
>     Graph:
> 
> 
> 
> 
> 
> 
>         On 07/10/2017 05:55 PM, Andy Walls wrote:
>         > From:         HLL
>         > Date:         Mon, 10 Jul 2017 20:44:01 +0300
>         >> Hi,
>         >> Thank you very much!!
>         >> I Need to thoroughly go over your response and understand it
>         all, but
>         >> thanks :)
>         >>
>         >> I also noticed the 2 different in bit timings, I thought it's
>         >> something electrically, since I noticed the "long" lows and
>         highs are
>         >> on some specific timings and the shorts have another timing.
>         >>
>         >> Before experimenting with the graph (and the said OOT
>         modules). I'm
>         >> going over it and trying to understand it,
>         >> what the rotator does, and what it it's role?
>         >
>         > It performs a (cyclic) frequency shift of the signal
>         spectrum.  It is
>         > called a rotator because the DFT of a sampled signal "lives"
>         on the
>         > unit circle of the z-plane.  The rotator block rotates the
>         entire z-
>         > plane about its origin by a certain number of radians, thus
>         effectively
>         >  shifting the spectrum of the signal.
>         >
>         > I use the rotator block to shift the audio frequency bins of
>         +350 Hz
>         > and +940 Hz down to -295 Hz and +295 Hz respectively.  Then I
>         filter
>         > off what were the negative audio frequency bins, the DC spike
>         from the
>         > FM CFO, and a lot of the spectrum which is just noise.
>         >
>         >> The part with 2 pll carrier tracking is used for locking the
>         carrier
>         >> of the low and high freq as I understand (I.E. The cheap
>         digital PWM
>         >> or clock devider)
>         >
>         > Yes, but they both track *and* downconvert the tracked tone to DC.
>         >
>         > This is a coherent FSK receiver design, which is probably
>         overkill for
>         > this application, but I used it to handle uncertainty in the
>         actual
>         > audio tone bins used for the mark and space frequencies.
>         >
>         >> what is the role of the complex conjugates (mirror over the real
>         >> axis?),
>         >
>         > The complex conjugate is to handle a quirk of the GNURadio PLL
>         block
>         > before the subtraction.  When the PLL carrier tracking block
>         does it's
>         > downconversion of the tracked tone to DC, it doesn't have a
>         phase angle
>         > of 0 degrees (a purely real number), instead it has a phase
>         angle of
>         > something a bit less than pi/4 radians.
>         >
>         > The complex conjugate is so when I do the following subtract,
>         I will
>         > get constellation points on opposite sides of the circle in
>         the I-Q
>         > plane.
>         >
>         >
>         >>  subtract,
>         >
>         > This is standard for a coherent FSK demodulator and for
>         certain non-
>         > coherent FSK demodulators.  Google images should show a number
>         of block
>         > diagrams doing this.
>         >
>         >
>         >>  c-to-f and add part?
>         >
>         > Well, after the subtraction you have I-Q plane constellation
>         points of
>         > about A*exp(j*pi/4) and A*exp(j*5*pi/4), and a fuzzy
>         trajectory line
>         > going approximately straight between those points.  I needed
>         to convert
>         > those to real values.
>         >
>         > I could have taken the complex magnitude and the complex
>         argument and
>         > somehow tried to assign the proper sign to the complex
>         magnitude, but
>         > that was work. :)  Since the two constellation points and the
>         > trajectory is restricted to quadrants I and III of the I-Q
>         plane, it
>         > was easier to just add combine the real and imaginary parts to
>         get a
>         > real number.
>         >
>         >
>         >>   Are you "subtracting" the (locked) `0` square wave from the `1`
>         >> square wave, why?
>         >
>         > No.
>         >
>         > Let's pretend GNURadio's quirky almost pi/4 angle output from the
>         > downconverted tone is actually 0 radians instead of almost pi/4.
>         >
>         > When the mark PLL is locked on to the mark tone, it will
>         output a value
>         > of A.  When the space PLL is locked on to a space tone, it
>         will output
>         > a value of A as well.
>         >
>         > To have a mark symbol represented by A and a space symbol
>         represented
>         > by -A, we have to invert the output value of the space PLL,
>         hence the
>         > subtraction.
>         >
>         > Also note, that one should not receive mark and space tones at
>         the same
>         > time, so when the mark PLL is outputting A, the space PLL is
>         ideally
>         > outputting 0, and vice-versa.
>         >
>         >
>         >> I think I understand most of the rest (the `missing block`
>         from their
>         >> names :) )
>         >>
>         >> Thanks,
>         >> HLL
>         >>
>         >> P.S. FYI, The capture I'v attached contains 4 bursts of 2
>         devices, 2
>         >> from device A and 2 from device B.
>         >
>         > Yes, I noticed 4 bursts, two at one energy level and two at
>         another
>         > energy level.
>         >
>         > Regards,
>         > Andy
>         >
>         >> P.S.2 It is probably some cheapo electronic components or
>         re-using
>         >> the micro that is already there.
>         >
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