Discussion thread for the Linux User Net

Topic for December 3, 2018 (KC7MM, NCS)

Receive and digitally process RF signals

• APRS
• Satellites: AMSATs, weather
• Computer-based spectrum analyser, panadapter
• Scanning bands
• Monitoring. The rtl_fm utility can be used as the start of a command-line chain of pipes (with sox and socat) that lets you tune to a station and then stream it over a network. Details here.

Start with a device that receives signals and digitizes them, and then inputs them to a computer (usually via USB).

• RTL-SDR
  • Based on the Realtek RTL2832U
  • 500 kHz – 1.7 GHz
  • Where to buy – on Amazon
  • Linux software: rtl-sdr package in Mint, contains rtl_fm utility (documented at link)
• FUNcube Pro+ Designed to work with the FUNcube satellite — an AMSAT-UK project (AO-73).
• SDRPlay
• HackRF One Transmits as well as receives.
• Airspy Comes with Linux software for multiple platforms, including Raspberry Pi. Servers located worldwide.
• NooElec

A first look at some client software.

• gqrx client software
  • Is in Mint repository
  • YouTube video page — there are lots of videos
    • APRS reception
  • Is basically a user interface; uses GNU Radio for radio functions (see below).

Performs all the signal processing for SDR.

• Main Web site
• Wiki main page
• Live Ubuntu bootable image for experimentation: GNU Radio Live SDR Environment
• GNU Radio Companion: a graphical tool for creating signal flow graphs and generating flow-graph source code (Python)
• Works well in Python programs.

Performs all the signal processing for SDR. Alternative to GNU Radio, written in the Lua scripting language.

• Main Web site
• Comparison to GNU Radio
• Reference manual

• Getting Started with GNU Radio and RTL-SDR on Linux (Kali) June 2012
• RTL-SDR FM Radio Receiver With GNU Radio Companion. [Instructables]
  • Detailed HOWTO
  • Author uses Arch
• Simple FM Band Spectrum Analyzer
• Quickstart SDR with gqrx, GNU Radio and the RTL-SDR dongle
  • Building an antenna and receiving ham and shortwave stations with SDR
    • HF bands
    • Requires an upconverter [NOTE: not needed with the current version of RTL-SDR.]
• Multi-channel RTL-SDR support from MATLAB, GNU Radio and Python Write a program for processing real-time radio signals.

Topic for January 7, 2019 (KC7MM, NCS)

I have been experimenting with an RTL-SDR dongle, using both the gqrx graphical application and terminal-based utilities contained in the rtl-sdr package (from the Mint repository) as the interface to it. Tonight's Net topic is about my experience so far.

This package contains a set of command line utilities:

• rtl_adsb: a simple ADS-B decoder for RTL2832 based DVB-T receivers
• rtl_eeprom: an EEPROM programming tool for RTL2832 based DVB-T receivers
• rtl_fm: a narrow band FM demodulator for RTL2832 based DVB-T receivers
• rtl_sdr: an I/Q recorder for RTL2832 based DVB-T receivers
• rtl_tcp: an I/Q spectrum server for RTL2832 based DVB-T receivers
• rtl_test: a benchmark tool for RTL2832 based DVB-T receivers

Homepage: http://sdr.osmocom.org/trac/wiki/rtl-sdr

After some manipulation of the TV module (detailed on the Web site), I was able to use rtl_fm to listen to the FM broadcast band with the command:

rtl_fm -f 89.9e6 -s 240000 -r 96000 - | aplay -r 96000 -f S16_LE

Changing the frequency of 89.9 to 147.04 got me the ARRG repeater at Sylvan.

gqrx is a graphical user interface for running SDR devices on Linux. I installed it from the Linux Mint repository. When I started up the program, it detected the RTL-SDR dongle, and I had no trouble selecting it and starting it going. There are many YouTube videos on using this application; search for gqrx.

gqrx Web site

For Ham Radio purposes, just listening to the signal might not be sufficient. For example one might want to send the received signal to fldigi to decode digital modes. That's not hard to do. I was able to get it working using the Pulse Audio Volume Control to link the input of fldigi to the output of gqrx. I learned how by watching YouTube videos by Kevin Loughlin, KB9RLW, which were most helpful:

• Ham Radio and Linux - The easy way to patch audio between applications
• Decoding signals from your SDR using FLDigi and Pulse Audio

I also succeeded in getting the signal into Audacity.

The rtl_tcp command-line utility will stream the SDR signal to a TCP port. I'd like to see about using that in place of gqrx as the signal source for fldigi. Pulse Audio won't work for that, so I'll try to use multimon-ng, as described in another KB9RLW video: Decoding POCSAG pager transmissions with GQRX and multimon-ng. As a practical application, I'm thinking that streaming an RTL-SDR might be a simple way for the base unit of a weather station I'm building to receive telemetry from remote sensors and route it to processing code I'll write in Python.