Rtl-sdr

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RTL-SDR is a driver that enables the use of Realtek RTL2832-series based DVB-T tuners as a cheap (<$25 USD) Software Defined Radio hardware.

Installation

Kernel

RTL-SDR is incompatible with the rtl2832 kernel driver. If the driver has been built as a module it must be blacklisted:

FILE /etc/modprobe.d/blacklist.conf
blacklist rtl2832

If the rtl2832 driver has been built into the kernel it must either be built as a module and blacklisted (if DVB functionality is desired), or not selected.

KERNEL Disable rtl2832 driver
Device Drivers --->
    <*> Multimedia support --->
        [*] Customise DVB Frontends --->
            <> Realtek RTL2832 DVB-T
Note
If DVB functionality is not desired it is safe to disable DVB entirely under Device Drivers ---> Multimedia support
Note
While the RTL-SDR driver is in use it is impossible to use the device as a DVB Tuner

Userspace

Install the net-wireless/rtl-sdr driver:

root #emerge --ask net-apps/rtl-sdr

Create the sdr group to allow non-root users to access the device:

root #groupadd sdr

Add any users that need to access the SDR device to the sdr group:

root #usermod -aG sdr larry
Note
If larry is currenlty logged in, he will have to log off for the group change to take effect.

Use lsusb to confirm the product and vendor IDs of the device:

user $lsusb
Bus 002 Device 002: ID 413c:81cc Dell Computer Corp.
 ...
Bus 001 Device 008: ID 0bda:2838 Realtek Semiconductor Corp. RTL2838 DVB-T

Add a udev rule to create the device /dev/rtl_sdr:

FILE /etc/udev/rules.d/20-rtlsdr.rules
SUBSYSTEM=="usb", ATTRS{idVendor}=="0bda", ATTRS{idProduct}=="2838", GROUP="sdr", MODE="0666", SYMLINK+="rtl_sdr"

Reload and trigger your udev rules:

root #udevadm control --reload-rules
root #udevadm trigger

Plug in the device and ensure that /dev/rdl_sdr is created by udev.

root #ls -la /dev/rtl_sdr
lrwxrwxrwx 1 root root 15 May 11 13:32 /dev/rtl_sdr -> bus/usb/001/007

Test the device using rtl_test.

user $rtl_test
Found 1 device(s):
 0:  Realtek, RTL2838UHIDIR, SN: 00000001

Using device 0: Generic RTL2832U OEM Found Rafael Micro R820T tuner

 ...

User cancel, exiting...

Samples per million lost (minimum): 0
Note
If root can perform this task but larry cannot, ensure that larry is a member of the sdr group and that he has logged off and back on since he was added to it.

Software

Install a package such as net-wireless/cubicsdr to gain a graphical view of the radio spectrum and easily capture tuned frequencies.

The rtl_power utility (installed as part of net-wireless/rtl-sdr) can be used to scan the spectrum available to the rtl2832 device (~64 - 1700MHz) for a period of time and output the results to a csv file. This can be converted into a waterfall graph that can be analysed to find interesting signals to investigate.

The example command below will scan from 13MHz to 1750MHz in bins of 200kHz, with a scan interval of 10 seconds for four hours:

user $rtl_power -f 13M:1750M:200k -i 100 -e 4h ~/sdr_data.csv

This can be converted to a waterfall using the script available here.