Sampling at 250 kHz without aliasing

Syed Karim syed at outernet.is
Sat Sep 26 18:50:10 UTC 2015


We have a need for a small sample rate on the rtl-sdr. We have been able to
select a bandwidth of 250 kHz, but there is significant aliasing at that
setting. At higher sample rates the aliasing no longer exists. We are using
the dongle that is sold by the RTL-SDR.com blog:

http://www.rtl-sdr.com/new-products-rtl-sdr-with-1ppm-tcxo-sma-f-connector-r820t2-bias-tee-improved-tolerances-direct-sampling-break-out-pads-now-available-in-our-store/


Since a reduced sample rate would allow for a much less resource
consumption by the CPU, we would gladly sponsor an open source bounty or
pay for open source development work to address this problem.

Our use case is reception of a 5 kHz channel from a geostationary L-band
operator. We have found that the dongle gets very hot at 1550 MHz, so we
will use an LNB to downconvert to ~400 MHz. The rest of the RF chain
consists of a patch antenna with 8 dBi of gain and 17 dBW EIRP from the
spacecraft.

The demodulator will be running on a derivative of the Beagle Bone Black.
The designs for this board are available here:

https://github.com/outernet-project/lantern-compute-pcb

You can learn more about Outernet through the following links.

http://gizmodo.com/what-is-the-outernet-and-is-it-the-future-of-the-intern-1659647614
http://www.wired.com/2015/07/plan-beam-web-3-billion-unconnected-humans/
http://www.technologyreview.com/news/537411/startup-beams-the-webs-most-important-content-from-space-free/
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