Fun with Software-Defined Radios: Mapping the Spectrum in the Mission, SF

https://medium.com/@BeepLabs/fun-with-software-defined-radios-d547c83a7492

by slewis • 11 years ago

67 14 11 years ago

fapjacks• 11 years ago

If the $650 pricetag for the hardware described in the post leaves a bad taste in your mouth, you can use a fifteen dollar (including shipping) SDR from China[0] for basically the same spectrum coverage and utility[1].

[0] http://www.amazon.com/RTL2832U-Low-Cost-Software-Compatible-... [1]: http://www.rtl-sdr.com/

npen2• 11 years ago

Hi, author here -- I agree, the BladeRF is overkill for this kind of application. But, we were done with what we needed it for and it was just sitting around our lab so we decided to use it. I did use an RTL-SDR to poke around in the lower half of UHF, though.

mspecter• 11 years ago

Also check out the HackRF One! You can look at a much wider spectrum, as well as broadcast. The thing itself is 300$, and completely open. (https://www.sparkfun.com/products/13001)

bigiain4• 11 years ago

Much narrower bandwidth though, and limited to below ~1700KHz - you can't snoop on WiFi/Bluetooth/upper-end-GSM-bands without futzing around with downconverters.

vvanders• 11 years ago

Love the penny antenna.

bigiain4• 11 years ago

I would have (in fact have done) gone for a diy discone or logperiodic antenna for wideband reception like this...

agoetz• 11 years ago

If you try to estimate power spectral density using the intuitive unbiased estimator (the DFT), you are going to have a bad time. A vanilla periodogram has very high sideband leakage, which means that the energy of the signal at a single frequency will look as if it "smeared out" across neighboring frequencies. The standard solution to this is to use the so-called 'modified periodogram' where the implicit rectangular window is replaced by a different windowing function with lower sideband leakage. In general, there is a direct tradeoff between sideband power and center frequency power, and in this application, you would do well to use a different window, such as the blackman-harris, or hamming window. See [1] for more details.

In addition even the modified periodogram discussed above has asymptotically nonzero variance [2], which means no matter how many samples you take, you will still have 'noise' in your PSD estimate. If you use biased estimators of the periodogram, such as the welch-bartlett method or the blackman-tukey algorithm, you will get much better results.

[1] http://www.ni.com/white-paper/4844/en/

[2] http://www.mathworks.com/help/signal/ug/nonparametric-method...

npen2• 11 years ago

Thank you for this! We'll do some more research and look into modifying spectrum_sense.

edit: After inspecting the code, spectrum_sense is indeed applying a blackman-harris window before the DFT.

throwacnt• 11 years ago

In theory, everything you write is correct.

In practice, none of it matter in this case. The leakage of rectangular window is ~30dB at a distance of 30 bins, and they aggregate more than 30 adjacent bins together ("re-bin"), making the leakage no more than 1 bin. To make things worse, the dynamic range of their receiver barely scrapes 40dB (peak) or 30dB (SFDR/SINR) - rendering the use of a more sophisticated window a moot point.

ChuckMcM• 11 years ago

I have been interested in this stuff since I saw a presentation on SDR by the folks at Ettus Research (and their amazing SDR setup). I eventually bought a HackRF One from Greatscott Gadgets to play with and it, like the BladeRF can cover a lot of ground with a pretty wide chunk (its only 20Mhz bandwidth generally but this has not been a limitation so far). So far I've been looking at beacons and other iOT type things, and decoding the remote control to my ceiling fan (fun). Very fun stuff to look at the spectrum and see what is there.

gbtw• 11 years ago

Essentially what you really want to build is the University of Twente SDR that's been up for a few years and you can listen in on over the web :)

http://websdr.ewi.utwente.nl:8901/

Fun with Software-Defined Radios: Mapping the Spectrum in the Mission, SF

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