RTL-SDR WITH LINRAD UNDER LINUX.
(Oct 27 2012)

The rtlsdr USB dongle.

Marketed as DVB-T LAPTOP DIGITAL TV Tuner USB Stick HDTV Realtek RTL2832U / E4000 SDR or similar one can find these surprisingly good receivers on the Internet for perhaps USD 20. They may look like the ones in figure 1.

Fig. 1. USB dongles for digital TV and FM radio.

For more info look here: http://sdr.osmocom.org/trac/wiki/rtl-sdr

The first version of Linrad that could be used with rtl-sdr was linrad-03.40. Look here RTL-SRD with Linrad-03.40 for performance tests.

The osmocom library is continously changing and linrad-03.40 to 03-44 can only use old versions of the library.

Starting with Linrad-03.45 there is a complete Linrad version of the rtl-sdr library. Linrad-03.45 works with the standard library and may continue to do so, but the Linrad version allows a much better usage of the hardware. The modifications are not elegant, but they are simple. The feature that Linrad adds is to allow different gain distributions for different usages. The modifications have been posted on the osmocom mailing list as a diff file but it seems maintainers are not interested.

Dynamic range.

Linrad allows four gain modes with rtl-sdr when the hardware uses the e4000 tuner chip.

  • 0. AGC mode.

  • 1. Compromise. This is the gain settings available in the rtl-sdr library as of October 2012.

  • 2. Linearity mode. For use without filters in environments with strong signals. Low front end gain and high gain after filters.

  • 3. Sensitivity mode. For use in rural locations or when filters and preamplifiers are placed between the antenna and the dongle. High front end gain and low gain after the filters.
The first version of Linrad to have selectable gain modes is linrad-03.45. The dynamic range performance in the three non AGC modes is listed in tables 1 to 3. The AGC mode does not work properly, at high levels the AGC oscillates. That should be easy to cure, the original rtl-sdr library june 2012 had a well working AGC mode. 

                    143.8           145.5             154.2
Gain  NF    MDS    Sat.  DR        Dgr.  DR         Dgr    DR
(dB) (dB)  (dBm)  (dBm) (dB)      (dBm) (dB)       (dBm)  (dB)
 42  7.9  -139.1   -63   76        -54   85         -53    86
 34 12.6  -134.4   -56   78        -45   89         -46    88
 29 12.6  -134.4   -56   78        -45   89         -46    88
 24 17.4  -129.6   -51   79        -40   90         -40    90
 22  19    -128    -49   79        -38   90         -38    90
 19  22    -125    -47   78        -35   90         -35    90  
 17  25    -122    -43   79        -32   90         -32    90
 14  27    -120    -42   78        -30   90         -30    90
 12  30    -117    -38   79        -27   90         -27    90
  9  32    -115    -37   78        -25   90         -25    90
  7  35    -112    -34   78        -22   90         -22    90
  4  37    -110    -31   79        -19   91         -19    91
  2  41    -106    -28   78        -16   90         -16    90
 -1  42    -105    -26   79        -14   91         -14    91
Table 1. The E4000 tuner. Performance of Linrad with the original OSMOCOM gain setting routines of Oct 2012. Gain mode 1 in Linrad. 


                    143.8           145.5             154.2
Gain  NF    MDS    Sat.  DR        Dgr.  DR         Dgr    DR
(dB) (dB)  (dBm)  (dBm) (dB)      (dBm) (dB)       (dBm)  (dB)
 25  8.3  -138.7   -66   73        -54   85         -30   109 
 20 12.5  -134.5   -60   75        -49   86         -25   110 
 15 15.5  -131.5   -56   76        -45   87         -23   109
 10  20    -127    -51   76        -40   87         -18   109
  5  26    -121    -46   75        -35   86         -13   108
  0  31    -116    -40   76        -29   87          -9   107
 -5  36    -111    -35   76        -24   87          -7   104
-10  38    -109    -31   78        -20   89          -6   103
-15  43    -104    -26   78        -15   89          -6    98
-20  47    -100    -21   79        -10   90          -6    94
-25  49     -98    -14   84        -10   88          -6    92
Table 2. The E4000 tuner. Performance of Linrad with the linearity gain setting, gain mode 2 in Linrad. This mode is suitable when very strong signals are present at frequencies 10 MHz or more away from the desired signal. Note the bold numbers in the table. 


                    143.8           145.5             154.2
Gain  NF    MDS    Sat.  DR        Dgr.  DR         Dgr    DR
(dB) (dB)  (dBm)  (dBm) (dB)      (dBm) (dB)       (dBm)  (dB)
 25  7.0   -140    -69   71        -58   82         -30   110 
 20  7.5  -139.5   -64   76        -53   87         -30   110
 15  8.6  -138.4   -59   79        -48   90         -30   108
 10 10.0  -137.0   -54   83        -43   94         -30   107
  5 15.0  -132.0   -49   83        -38   94         -27   105
  0  17    -130    -44   86        -36   94         -26   104
 -5  23    -125    -40   85        -34   91         -23   102
-10  28    -120    -35   85        -33   87         -19   101
-15  32    -116    -29   87        -29   87         -15   101
-20  37    -111    -27   84        -24   87         -12    99
-25  41    -107    -26   81        -26   81          -7   100
Table 3. The E4000 tuner. Performance of Linrad with the sensitivity gain setting, gain mode 3 in Linrad. This mode is suitable when a filter is used to eliminate signals outside the band of interest.


The data delivered from the rtlsdr is only 8 bit. An ideal 8-bit A/D converter should have a SNR of 50 dB The bandwidth delivered by the rtlsdr is 2 MHz while we measure MDS in a bandwidth of 500 Hz. The bandwidth factor is 4000 and that provides a gain of 36 dB. Note that the dongle reaches the theoritical limit in sensitivity mode at gain settings of 0 to -15 dB. Measurements have to be made with a third signal present because otherwise the A/D converter might give the same value most of the time. With a GAAS FET LNA having the gain required to lift the noise floor by 3 dB in sensitivity mode with 0 dB gain setting and with adequate filtering one can get very good performance with the E4000 dongle. With a 1 MHz wide filter at i.e. 432 MHz one can get a dynamic range near 86 dB with a sensitivity only about 3 dB worse than optimum. By increasing the gain to 15 dB one would then have a sensitivity very close to the theoretical limit at a dynamic range of about 68 dB. Note that LO spurs and reciprocal mixing can be the limiting factors.

Installing librtlsdr.

The library can only be installed under Windows when this is written (Oct 2012.) To install the current Osmocom library for rtl-sdr one can use the following set of commands: 
   rm -r /usr/src/rtl-sdr*
   cd /usr/src
   git clone git://git.osmocom.org/rtl-sdr.git
   mkdir build
   cd build
   cmake ../
   make
   make install
Two routines need to be changed to get the results demonstrated on this page: librtlsdr.c and tuner_e4k.c. October 18 2012 they differ from the original library as can be seen in this diff file: linrad-oct18.diff (10267 bytes) The diff might be useful to get the Linrad modifications into newer versions of the library. The source code of the modified library as of Oct 18 2012 is contained here rtl-sdr-linrad1.tbz (875606 bytes)

The library at osmocom is improving over time. Here is the source code with the modifications applied to the latest osmocom code by January 26 1013. rtl-sdr-linrad2.tbz (423422 bytes)