How Simple Radio Broadcasting and Receiving Work Using SDR
电子科技大学 格拉斯哥学院 2017级 李文翔
Abstract
Radio waves has become a very important medium for information transmission, as it can be modified easily and it transmits in the speed of light, which means that we can always receive the newest message and it changed the means of communication completely. SDR is a radio device which works with the help of software and in this article, I’ll discuss some basic functions of SDR and how it improve our daily life.
1. What is radio wave.
Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies as high as 300 gigahertz (GHz) to as low as 30 hertz. At 300 GHz, the corresponding
wavelength is 1 mm, and at 30 Hz is 10,000 km. Like all other electromagnetic
waves, radio waves travel at the speed of light. They are generated by electric
charges undergoing acceleration, such as time varying electric currents.
Naturally occurring radio waves are emitted by lightning and astronomical objects.
Radio waves are generated artificially by transmitters and received by radio receivers, using antennas. Radio waves are very widely used in modern technology for fixed and mobile radio communication, broadcasting, radar and other navigation systems,
communications satellites, wireless computer networks and many other
applications.
Radio waves in vacuum travel at the speed of light. When passing through a material medium, they are slowed according to that object’s permeability and permittivity. Air is thin enough that in the Earth’s atmosphere radio waves travel very close to the
speed of light.
We can use the method of amplitude modulation and frequency modulation to add the information we need into the radio waves. When a receiver gets the radio wave, it can analyses the message contained in the wave so that we can get the information accurately and swiftly.
So with the help of radio waves, we can have our information transmitted at a very fast
speed, and thus shorten the time delay while communication. Beside this, radio
wave transmission can be done even without medium, so this makes communication
between spaces and ground possible.
2. What is an SDR
Software-defined radio (SDR) is a radio communication system where components that have been traditionally implemented in hardware (e.g. mixers, filters, amplifiers, modulators/demodulators, detectors, etc.) are instead implemented by means of software on a personal computer or embedded system. While the concept of SDR is not new, the rapidly evolving capabilities of digital electronics render practical many processes which were once only theoretically possible.
In our case, the SDR we use is called Mini Digital TV Stick (DVB-T+FM+DVB 820T2 &
SDR), which is consisted of three main parts: a TV stick with USB socket, an antenna, and an antenna socket which can be connected to the TV stick. A remote is added in order to control the SDR in distance, while it is seldomly used.
Although it is defined as a TV stick, which is used to turn your computer into a TV whose information is propagated via the radio wave, it can be used as a radio receiver to receive other radio signals, such as the radio that we always listen to in cars, which is one of the most simple function it has. With proper setup and correct software, it can be done without much efforts.
3. How an SDR works [^1]
Ideal concept
The ideal receiver scheme would be to attach an analog-to-digital converter to an antenna. A digital signal processor would read the converter, and then its software would transform the stream of data from the converter to any other form the application requires.
An ideal transmitter would be similar. A digital signal processor would generate a stream of numbers. These would be sent to a digital-to-analog converter connected to a radio antenna.
The ideal scheme is not completely realizable due to the current limits of the technology. The main problem in both directions is the difficulty of conversion between the digital and the analog domains at a high enough rate and a high enough accuracy at the same time, and without relying upon physical processes like interference and electromagnetic resonance for assistance.
Receiver architecture
Most receivers use a variable-frequency oscillator, mixer, and filter to tune the desired signal to a common intermediate frequency or baseband, where it is then sampled by the analog-to-digital converter. However, in some applications it is not necessary to
tune the signal to an intermediate frequency and the radio frequency signal is
directly sampled by the analog-to-digital converter (after amplification).
Real analog-to-digital converters lack the dynamic range to pick up sub-microvolt,
nanowatt-power radio signals. Therefore, a low-noise amplifier must precede the
conversion step and this device introduces its own problems. For example, if
spurious signals are present (which is typical), these compete with the desired
signals within the amplifier’s dynamic range. They may introduce distortion in
the desired signals, or may block them completely. The standard solution is to
put band-pass filters between the antenna and the amplifier, but these reduce
the radio’s flexibility. Real software radios often have two or three analog
channel filters with different bandwidths that are switched in and out.
4. How to use SDR to receive radio waves.
To receive radio wave using SDR is one of the simplest applications of SDR, as
there’s no need to modify the hardware of the SDR, only need to add software to
the personal computer so that it can work properly. The software, which is
called Zadig, is a software that install the USB driver, as for the SDR, it
doesn’t contain any installation package.
After successfully installed the driver, the SDR now turns into a radio wave receiver
and is working the moment you connect it to your computer. However, without any
software that shows the radio wave, we no nothing about it. So that we need
software that shows the wave, which can be done by SDR sharp, which is a
software that shows all frequency range that the receiver receives. The peak of
the gain can be seen on the screen, which is the signal that is strong in the area.
This could be local radio broadcasting by the radio station, or if you are
lucky, you can get the frequency of the walky-talky of the patrol personal.
However, this device can only be acted as a receiver, how can we transmit our voice
message and receive by our SDR? There is a simple solution, which is the same
as how the radio station broadcast their information. We need a device, which acts
like a transceiver, and in my case, I bought an FM stereophonic transmitting
module. It is a small DIY device that can broadcast radio wave in 14 different
frequencies inside a range of 15m indoor, and it has a decent performance and I
can receive my message through the SDR.
5. Simple SDR function in daily life.
In our daily life, SDR can be a very useful tool. The most obvious one is that it
turns your computer into a radio or even a satellite TV (if it is available).
Compared to the traditional radio and TV, they are too large, and the functions
are limited. However, with the portable SDR, we can listen to the radio and
watch TV on the computer while working on it. And we can even turn our mobile
phone into a radio, as phones are seldom integrated with radio module nowadays.
Besides this, communication can be easier and safer when neighbours talking to
each other, as your chatting log can be obtained by hackers if you are
communicating via the Internet and it can be expensive if talking using the
mobile phone. A pair of transceiver and SDR can provide an environment that not
only cost less but also they can guarantee safety to some extent.
6. SDR application in other fields. [^1]
A typical amateur software radio uses a direct conversion receiver. Unlike direct conversion receivers of the more distant past, the mixer technologies used are based on the quadrature sampling detector and the quadrature sampling exciter.
The receiver performance of this line of SDRs is directly related to the dynamic range of the analog-to-digital converters (ADCs) utilized. Radio frequency signals are down converted to the audio frequency band, which is sampled by a high performance audio frequency ADC. First generation SDRs used a PC sound card to provide ADC functionality. The newer software defined radios use embedded high performance ADCs that provide higher dynamic range and are more resistant to noise and RF interference.
The SDR software performs all of the demodulation, filtering (both radio frequency and audio frequency), and signal enhancement (equalization and binaural presentation). Uses include every common amateur modulation: morse code, single sideband modulation, frequency modulation, amplitude modulation, and a variety of digital modes such as radioteletype, slow-scan television, and packet radio. Amateurs also experiment
with new modulation methods: for instance, the DREAM open-source project
decodes the COFDM technique used by Digital Radio Mondiale.