Wednesday, June 3, 2015

New Way to Feed Radio-Sky Spectrograph with a Dongle Receiver

One of the limitations of the RTLSDR dongle receivers is that it is hard to get more than about 2.4 MHz of bandwidth  from them.  Raydel Abreu Espinet, CM2ESP,  has written a small Windows application that replaces RTL Bridge by speaking directly to Radio-Sky Spectrograph (RSS) .  What Raydel's software does that RTL Bridge is not capable of is stacking sequential spectra so that you can view a swath of spectrum  even tens of MHz wide.  The program is called RTL-WideSpectrum (RTLW) and does its magic by simply stepping through a number of adjoining spectra and combing their outputs into the modest number of channels permitted by RSS, (512 max). It does this very quickly by working more directly to the RTL dongle than the method used by RTL Bridge ( which uses rtltcp.exe as an intermediary ). 

Above is a RSS spectrogram of a solar burst recorded by Raydel from 28 to 48 MHz.  The horizontal banding is due to the sensitivity roll off at the edges of the 2 MHz band pass of the RTL Dongle.  This can be compensated for to some degree by using the Normalize button in RSS.  I don't know what kind of windowing RTLW is using but there may be some window function that could produce a more even spectrum.

You need to install SDR# and the Zadig driver to use Raydel's app. A Quick Start Guide is available here. Also of course you need RSS which can be downloaded here:

After installing RSS, you can download and run RTLW. I have placed a copy on my website.  Raydel may wish to put future versions somewhere else. Download and unzip the folder to someplace you can find it.  There is no install procedure.  You might want to create a shortcut to the exe file.

Plug in your dongle and try it out with SDR# first using SDR#'s own drivers.  Once you are satisfied that it is working, run RTL-WideSpectrum.exe from wherever you installed it., 

 At first you need to select a gain and then a frequency correction.  Finally you must select a total spectral width  (5,10,or 20 MHz), and a center frequency for the lowest frequency spectrum.

Run RSS in stand alone mode and select Options /Radio /RTL dongle receiver.  Then press the Start button in RSS  The charts should start scrolling.  If the signal looks too hot, stop the chart and restart RTLW with less gain. I am using 28 for the gain.

  Raydel says that there may be some updates coming and he is aware that there are some things that could be done to make the program easier to use but what he has provided is very workable and probably as effective as you will get using this method to increase the bandwidth of the RTL generated spectra. Obviously, I would love to make RTL Bridge a better performer by using some of Raydel's methods, but I am currently just too busy to work on that project and hope you will enjoy CM2ESP's innovative work.

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