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

[Cinaed Simson]

On 07/12/2017 05:39 PM, Cinaed Simson wrote:
> 
> 
> On 07/12/2017 03:56 AM, HLL 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
> 
> At least the ones in the US operate between at 4 frequencies between
> 450-470 MHz.
> 
> The frequency band will vary with country.
> 
>>
>>
>>     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 wavelength for 450 is 1.5 meters. I really don't know where the
> boundary is between near field and far field - but I would guess far
> field the other side of a full wave length.

Opps - Andy Walls I had the numbers upside down - should have checked
the units.

In any case, the wave length is 2/3 of meter and not 1.5 meters.
> 
> 
>> The antenna used is the stock RTL SDR one.
> 
> It's quarter wave antenna I believe for around 800-900 MHz.
> 
>>
>> If you can recommend some hackish (DIY or even retail) antenna to better
>> receive that signal that would be grate.

Actually, my PCB antennas are log periodic - they don't need a external
ground.

The problem with these antennas is need to build a stand. Probably not
worth the cost.

I would just put a large piece of metal under the antenna - quarter wave
antennas need a good ground - dirt is poor conductor. But then power is
116 dBm - you could probably use a paper clip soldered into a SMA
connector :).

> 
> Let me think about it. I use a RTL dongle from NooElec which has a SMA
> connector.
> 
>>
>>
>>     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
> 
> Actually, that should read 7-12.5 kHz. If I was doing it, and the
> documents indicated it uses a channel between 7-12.5 KHz, then I would
> sample at 64 kHz.
> 
> -- Cinaed
> 
> 
>>
>>
>>     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:
>>
>> Inline image 1
>>  
>> now it seems that a large CFO glitch translates to short glitchs in the
>> digital signal
>>
>> Graph:
>> Inline image 2
>>
>>
>>
>>
>>
>>     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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