Linux Format - UK (2020-03)

78 LXF260 March 2020 http://www.linuxformat.com

TUTORIALS Process radio signals

transmitter and/or an RF receiver. These devices are

referred to as SDR transceivers. For this tutorial we will

use a SDR dongle that is a receiver only – no transmit.

The objective is to extract the audio signal in an over-

the-air FM-broadcasted signal.

What is SDR good for?

With an antenna and the SDR dongle the sky is the limit.

There are different frequency ranges of RF signals. If the

antenna is tuned to the receive frequency and the SDR

is designed to operate in the same frequency range,

then the signal can be processed.

The software on the computer must then have the

capability of demodulating the signal and possibly

decoding the signal. Decoding suggests the signal was

encrypted and would need to be decrypted to be usable.

Here’s a short list of RF signals and the data that can

be extracted using an inexpensive SDR device:

AM/FM radio stations

Aviation communications (conversations between

aircraft and air traffic control)

Ham radio (person-to-person conversations)

Aircraft Automatic Dependent Surveillance-

Broadcasts (aircraft positions)

Satellite transmissions (weather data)

There is even a suggestions of recording vehicle

keyless entry broadcasts and then playing them back.

his tutorial is designed to explore software-

defined radio (SDR). We will cover some

radio frequency (RF) theory in order to have

a foundation for the article. The goal is to configure

an SDR USB dongle, for this tutorial on a Pi, to receive a

broadcasted FM radio signal.

SDR is a radio frequency communications system

built using software, rather than physical hardware

circuits consisting of oscillators, filters, modulators/

demodulators and signal amplifiers, to transmit or

receive a radio frequency broadcast signal.

SDR devices receive analogue radio signals through

an antenna, and then using an analogue-to-digital

converter (ADC) they digitise the signal that can be

further processed using digital signal processing.

What is in an RF broadcast?

An RF broadcast is a transmitted carrier signal travelling

through the air. The signal is encoded with information

using a modulation method. Two common RF

modulation methods are frequency modulation (FM)

and amplitude modulation (AM). In FM the carrier signal

frequency is up-shifted and down-shifted in carrier

frequency relative to the data applied. In AM the carrier

signal amplitude varies according to the data applied.

To extract the data from the RF signal you would

require a receiver that matches the modulation

technique the signal was broadcasted with. The

receiver has specific electronic circuits to process

the modulated signal. The signal is gathered at the

antenna and sent through the receiver circuit to extract

the data and produce an output.

The block diagram (see top right) presents a pictorial

of the electronic circuits needed to provide the

functions required inside the AM and FM radios to

produce the data. SDR replaces a majority of the

hardware by performing the bulk of the circuit-heavy

lifting in software. It also has the advantage of

performing this processing over a larger frequency

range than typical radio receivers.

With an antenna and an SDR dongle, radio signals

can be gathered via the antenna, digitised by the dongle

and processed using software on the Pi to produce the

data that was carried on the modulated carrier signal.

There are SDR dongles that can be both an RF

Receive and process

global radio signals

Sean Conway provides instructions on software-defined radio (SDR) that

turns your Raspberry Pi into a FM radio receiver.

Sean Conway’s

career started in

radio frequency

electronics but

later took a turn

into data

communication

and computer

systems.

T

CUBICSDR

OUR

EXPERT

Mixing and

matching three

antennas and two

dongles for testing.

Hardware

Raspberry Pi

3B+

16GB micro

SD card

5VDC 2.5 amp

power supply

USB micro

Software

Latest

Raspbian

Buster full

rtl-sdr 0.6-1

gqrx 2.11.5

cubicSDR

0.2.5

YOU NEED

78 LXF260 March 2020 http://www.linuxformat.com

TUTORIALS Process radio signals

transmitterand/oranRFreceiver.Thesedevicesare

referredtoasSDRtransceivers.Forthistutorialwewill

usea SDRdonglethatisa receiveronly– notransmit.

Theobjectiveistoextracttheaudiosignalinanover-

the-airFM-broadcastedsignal.

WhatisSDRgoodfor?

WithanantennaandtheSDRdongletheskyisthelimit.

TherearedifferentfrequencyrangesofRFsignals.If the

antennaistunedtothereceivefrequencyandtheSDR

isdesignedtooperateinthesamefrequencyrange,

thenthesignalcanbeprocessed.

Thesoftwareonthecomputermustthenhavethe

capabilityofdemodulatingthesignalandpossibly

decodingthesignal.Decodingsuggeststhesignalwas

encryptedandwouldneedtobedecryptedtobeusable.

Here’sa shortlistofRFsignalsandthedatathatcan

beextractedusinganinexpensiveSDRdevice:

AM/FMradiostations

Aviationcommunications(conversationsbetween

aircraftandairtrafficcontrol)

Hamradio(person-to-personconversations)

AircraftAutomaticDependentSurveillance-

Broadcasts(aircraftpositions)

Satellitetransmissions(weatherdata)

There is even a suggestions of recording vehicle

keyless entry broadcasts and then playing them back.

histutorialisdesignedtoexploresoftware-

definedradio(SDR).Wewillcoversome

radiofrequency(RF)theoryinordertohave

a foundationforthearticle.Thegoalistoconfigure

anSDRUSBdongle,forthistutorialona Pi,toreceivea

broadcastedFMradiosignal.

SDRisa radiofrequencycommunicationssystem

builtusingsoftware,ratherthanphysicalhardware

circuitsconsistingofoscillators,filters,modulators/

demodulatorsandsignalamplifiers,totransmitor

receivea radiofrequencybroadcastsignal.

SDRdevicesreceiveanalogueradiosignalsthrough

anantenna,andthenusingananalogue-to-digital

converter(ADC)theydigitisethesignalthatcanbe

furtherprocessedusingdigitalsignalprocessing.

WhatisinanRFbroadcast?

AnRFbroadcastisa transmittedcarriersignaltravelling

throughtheair.Thesignalisencodedwithinformation

using a modulation method. Two common RF

modulation methods are frequency modulation (FM)

and amplitude modulation (AM). In FM the carrier signal

frequency is up-shifted and down-shifted in carrier

frequency relative to the data applied. In AM the carrier

signal amplitude varies according to the data applied.

To extract the data from the RF signal you would

require a receiver that matches the modulation

technique the signal was broadcasted with. The

receiver has specific electronic circuits to process

the modulated signal. The signal is gathered at the

antenna and sent through the receiver circuit to extract

the data and produce an output.

The block diagram (see top right) presents a pictorial

of the electronic circuits needed to provide the

functions required inside the AM and FM radios to

produce the data. SDR replaces a majority of the

hardware by performing the bulk of the circuit-heavy

lifting in software. It also has the advantage of

performing this processing over a larger frequency

range than typical radio receivers.

With an antenna and an SDR dongle, radio signals

can be gathered via the antenna, digitised by the dongle

and processed using software on the Pi to produce the

data that was carried on the modulated carrier signal.

There are SDR dongles that can be both an RF

Receive and process

global radio signals

SeanConwayprovides instructions on software-defined radio (SDR) that

turnsyourRaspberry Pi into a FM radio receiver.

SeanConway’s
career started in
radio frequency
electronics but
later took a turn
into data
communication
and computer
systems.

T

CUBICSDR

OUR

EXPERT

Mixing and

matching three

antennas and two

dongles for testing.

Hardware

RaspberryPi

3B+

16GBmicro

SDcard

5VDC 2.5 amp

power supply

USB micro

Software

Latest

Raspbian

Busterfull

rtl-sdr0.6-1

gqrx2.11.5

cubicSDR

0.2.5

YOU NEED