http://www.techradar.com/pro/linux March 2020 LXF260 79Process radio signals TUTORIALS
ORIGINS OF SDR
This tutorial is intended for educational purposes.
In most countries it is legal to receive broadcast RF
signals, but in some countries this activity may be
illegal: it is your responsibility to be aware of any legal
restrictions in your region.How does SDR work?
Broadcasted RF signals that are gathered by the
antenna are converted to lower frequencies found in the
audio band with the compact electronics in the SDR
dongle. The frequency is then sampled using high-
performance analogue-to-digital converters (ADC) to
produce a digital output.
To paraphrase Nyquist’s theorem: an analogue signal
can be reconstructed if samples are taken at equal time
intervals. The sample rate must be equal to or greater
than the highest frequency being sampled. The ADC
used in SDRs have sample rates typically in the 200kHz
range – plenty to ensure an error-free reproduction.
We’ve chosen to ignore nearly all of the concepts and
complex maths associated with the operation of an SDR
device. Quadrature phase modulation theory used to
bend vectors 90 degrees at 0Hz is way beyond this
writer’s understanding, let alone explaining it effectively.What is needed for SDR?
The puzzle pieces required to assemble an SDR receiver
can be divided into three categories: the SDR device
itself; the antenna for receiving signals; and the
software, along with the PC it will run on. Components
from all three categories work together to produce an
output. Reducing the performance of any one of these
components will impact a successful SDR project. You
can have a smoking-hot performance from the SDR
dongle, but if the antenna doesn’t produce enough
input signal, the SDR may only produce the sound of
static noise.
There is a balancing act in the form of cost verses
performance, especially for the novice with little money
who wants to dabble in SDR. Given sufficient money, all
three categories could be optimised to achieve the best
performance. For the basement technology geek in
most of us, unlimited cost is not realistic. Our Quick Tips
provide some resources to help you decide what
components from the categories fit your budget.
Two dongles were used to develop this tutorial. The
price point of each was different. As much as we’d like
to purchase a higher-end SDR unit, clearer heads had to
prevail and only invest in purchases that were needed to
provide a comparison to complete the article.
In addition to the price of the two SDRs being
different, so was their performance. The cheaper SDR
dongle didn’t appear to work initially. After successfully
getting the more expensive SDR device working, we
doubled back using the knowledge gained to optimise
the software configuration, enabling audio to be
produced from the economical unit.
SDR receivers or SDR transceivers are designed to
span a specific frequency range. An SDR will have a
specification for the frequency range the device is
capable of handling. The range can be from low
frequencies in the kHz to frequencies in the GHz or
thousands of MHz range.
Devices like SDRs that are capable of transmitting
and receiving RF signal require an antenna to handle themodulated signal. The diagram (see page 81) shows an
ideal vertical dipole antenna radiation pattern. The RF
source sends a signal to the antenna and it is radiated
out in a torus or doughnut pattern. The RF signal energy
is equal in all directions.
Antennas are designed to provide the best
performance at a given frequency or range. The physical
construction of the antenna and the components used
determine the performance of the antenna. Antennas
can be designed to increase signal strength in one
direction with a reduction of the signal strength in other
directions. These antennas are directional.
The RF energy is focused in one direction with very
little escaping out the back. This type of antenna design
enables greater range in the direction the antenna is
pointed. If the signal is coming from the back side of the
antenna, performance is degraded.
At lower-frequency ranges the antenna length is
longer, sometimes kilometres long. In higher-frequency
ranges the antenna is shorter – think of a mobile phone.
An antenna tuned to a specific frequency provides the
best signal receive and transmit strength. If you
consider how signals are received on the Earth from
satellites transmitting while orbiting in space, it might
provide some appreciation for antenna design providing
maximum signal strength.
Not all SDRs are the same. SDR components range
from as little as $10 to $300 and more, depending on
the specifications. Most can be purchased as a kit and
come with wideband antennas. Wideband antennas are
designed to work across a larger frequency range and
tend to produce an average signal output across the
range. Recall antennas are tuned to receive or transmitSDR originates from a discovery made about an inexpensive mass-
produced DVB-T TV tuner dongle by Antti Palosaari, Eric Fry and
Osmocom (in particular Steve Markgraf). DVB-T is an abbreviation for
Digital Video Broadcasting-Terrestrial, one standard for digital
television broadcasting first published in 1997. The standard defines
frequency in the VHF and UHF RF frequency bands and how data is
carried on the RF signal.
The pioneers of SDR discovered something unique about the
Realtek RT2823U coded orthogonal frequency division multiplexing
chipset. The raw I/Q data on the RTL2832U chipset could be
accessed directly, making the DVB-T tuner a wideband software-
defined radio. The custom driver software developed by Steve
Markgraf was the keystone to SDR development.
Osmocom is an open source mobile communication project that
develops software and tools for mobile communications. It is the
maintainers of the RTL-SDR driver code today.Complex electronic circuit to support AM/FM radio.You’ll find a
list of RTL-SDR
supported
software
here: http://www.
rtl-sdr.com/
big-list-rtl-
sdr-supported-
software.Go to http://www.
rtl-sdr.com/
RTL-SDR for
the software-
defined radio
news and info.Audio
Discriminator amplifier
Amplifier/Filter
(10.7MHz)FM Tuner
(88-108MHz)AM Tuner
(540-1605KHz)Audio
amplifierAmplifier/Filter
(455KHz) Detector7778March 2 h8r0povh0oidesn March 2020 LXF260 79
Process radio signals TUTORIALS
ORIGINS OF SDR
This tutorial is intended for educational purposes.
In most countries it is legal to receive broadcast RF
signals, but in some countries this activity may be
illegal: it is your responsibility to be aware of any legal
restrictions in your region.
How does SDR work?
Broadcasted RF signals that are gathered by the
antenna are converted to lower frequencies found in the
audio band with the compact electronics in the SDR
dongle. The frequency is then sampled using high-
performance analogue-to-digital converters (ADC) to
produce a digital output.
To paraphrase Nyquist’s theorem: an analogue signal
can be reconstructed if samples are taken at equal time
intervals. The sample rate must be equal to or greater
than the highest frequency being sampled. The ADC
used in SDRs have sample rates typically in the 200kHz
range – plenty to ensure an error-free reproduction.
We’ve chosen to ignore nearly all of the concepts and
complex maths associated with the operation of an SDR
device. Quadrature phase modulation theory used to
bend vectors 90 degrees at 0Hz is way beyond this
writer’s understanding, let alone explaining it effectively.
What is needed for SDR?
The puzzle pieces required to assemble an SDR receiver
can be divided into three categories: the SDR device
itself; the antenna for receiving signals; and the
software, along with the PC it will run on. Components
from all three categories work together to produce an
output. Reducing the performance of any one of these
components will impact a successful SDR project. You
can have a smoking-hot performance from the SDR
dongle, but if the antenna doesn’t produce enough
input signal, the SDR may only produce the sound of
static noise.
There is a balancing act in the form of cost verses
performance, especially for the novice with little money
who wants to dabble in SDR. Given sufficient money, all
three categories could be optimised to achieve the best
performance. For the basement technology geek in
most of us, unlimited cost is not realistic. Our Quick Tips
provide some resources to help you decide what
components from the categories fit your budget.
Two dongles were used to develop this tutorial. The
price point of each was different. As much as we’d like
to purchase a higher-end SDR unit, clearer heads had to
prevail and only invest in purchases that were needed to
provide a comparison to complete the article.
In addition to the price of the two SDRs being
different, so was their performance. The cheaper SDR
dongle didn’t appear to work initially. After successfully
getting the more expensive SDR device working, we
doubled back using the knowledge gained to optimise
the software configuration, enabling audio to be
produced from the economical unit.
SDR receivers or SDR transceivers are designed to
span a specific frequency range. An SDR will have a
specification for the frequency range the device is
capable of handling. The range can be from low
frequencies in the kHz to frequencies in the GHz or
thousands of MHz range.
Devices like SDRs that are capable of transmitting
and receiving RF signal require an antenna to handle the
modulated signal. The diagram (see page 81) shows an
ideal vertical dipole antenna radiation pattern. The RF
source sends a signal to the antenna and it is radiated
out in a torus or doughnut pattern. The RF signal energy
is equal in all directions.
Antennas are designed to provide the best
performance at a given frequency or range. The physical
construction of the antenna and the components used
determine the performance of the antenna. Antennas
can be designed to increase signal strength in one
direction with a reduction of the signal strength in other
directions. These antennas are directional.
The RF energy is focused in one direction with very
little escaping out the back. This type of antenna design
enables greater range in the direction the antenna is
pointed. If the signal is coming from the back side of the
antenna, performance is degraded.
At lower-frequency ranges the antenna length is
longer, sometimes kilometres long. In higher-frequency
ranges the antenna is shorter – think of a mobile phone.
An antenna tuned to a specific frequency provides the
best signal receive and transmit strength. If you
consider how signals are received on the Earth from
satellites transmitting while orbiting in space, it might
provide some appreciation for antenna design providing
maximum signal strength.
Not all SDRs are the same. SDR components range
from as little as $10 to $300 and more, depending on
the specifications. Most can be purchased as a kit and
come with wideband antennas. Wideband antennas are
designed to work across a larger frequency range and
tend to produce an average signal output across the
range. Recall antennas are tuned to receive or transmitSDR originates from a discovery made about an inexpensive mass-
produced DVB-T TV tuner dongle by Antti Palosaari, Eric Fry and
Osmocom (in particular Steve Markgraf). DVB-T is an abbreviation for
Digital Video Broadcasting-Terrestrial, one standard for digital
television broadcasting first published in 1997. The standard defines
frequency in the VHF and UHF RF frequency bands and how data is
carried on the RF signal.
The pioneers of SDR discovered something unique about the
Realtek RT2823U coded orthogonal frequency division multiplexing
chipset. The raw I/Q data on the RTL2832U chipset could be
accessed directly, making the DVB-T tuner a wideband software-
defined radio. The custom driver software developed by Steve
Markgraf was the keystone to SDR development.
Osmocom is an open source mobile communication project that
develops software and tools for mobile communications. It is the
maintainers of the RTL-SDR driver code today.Complex electronic circuit to support AM/FM radio.You’llfinda
listofRTL-SDR
supported
software
here:www.
rtl-sdr.com/
big-list-rtl-
sdr-supported-
software.Gotowww.
rtl-sdr.com/
RTL-SDRfor
thesoftware-
definedradio
newsandinfo.Audio
Discriminator amplifier
Amplifier/Filter
(10.7MHz)FM Tuner
(88-108MHz)AM Tuner
(540-1605KHz)Audio
amplifierAmplifier/Filter
(455KHz) Detector