On Wed, Jul 12, 2017 at 1:56 AM, Cinaed Simson <cinaed.sim...@gmail.com> 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: [image: Inline image 1] now it seems that a large CFO glitch translates to short glitchs in the digital signal Graph: [image: 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. > > > > _______________________________________________ > > Discuss-gnuradio mailing list > > Discuss-gnuradio@gnu.org > > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >
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