12 May
2016
12 May
'16
11:47 p.m.
Hello Syed,
If you use Linrad, the narrowest IF bandwidth you can select
is currently 300 kHz with the R820T2. If you select the sampling
speed 300 kHz you will get an alias-free range of 200 kHz. The
stop band attenuation is about 30 dB.
If you select 300 kHz IF and 230 kHz sampling rate, the alias free range is
about 170 kHz. (-30 dB) You should be able to select a frequency
that avoids any strong alias on your desired signal frequency.
Linrad has its own library for rtlsdr dongles. Maybe, today, some
of the functionality is present in the Osmocom library, I have
not worked with thise things for quite some time. The R820T and
the R820T2 need to be treated differently, but that is not included,
my mods to the lib were made before the R820T2 arrived.
Linrad is here: http://www.sm5bsz.com/linuxdsp/linrad.htm
Regards
Leif
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-…
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-…
http://www.wired.com/2015/07/plan-beam-web-3-billion-unconnected-humans/
http://www.technologyreview.com/news/537411/startup-beams-the-webs-most-imp…