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 <[email protected] >> <mailto:[email protected]>> 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. >> > >> > _______________________________________________ >> > Discuss-gnuradio mailing list >> > [email protected] <mailto:[email protected]> >> > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >> <https://lists.gnu.org/mailman/listinfo/discuss-gnuradio> >> > >> >> >> _______________________________________________ >> Discuss-gnuradio mailing list >> [email protected] <mailto:[email protected]> >> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >> <https://lists.gnu.org/mailman/listinfo/discuss-gnuradio> >> >> >> >> >> _______________________________________________ >> Discuss-gnuradio mailing list >> [email protected] >> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >> > _______________________________________________ Discuss-gnuradio mailing list [email protected] https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
