On Wed, Jul 12, 2017 at 1:56 AM, Cinaed Simson <address@hidden> wrote:
![Inline image 1]()
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
![Inline image 2]()
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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