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Receipt of radio signals using RTL-SDR


The number of vibrations per second is measured in hertz.

1 kHz = 1000 Hz (one kilohertz is one thousand hertz)

1 MHz = 1,000 kHz = 1,000,000 Hz (one megahertz - one thousand kilohertz - one million hertz)

1 GHz = 1000 MHz = 1,000,000 kHz = 1,000,000,000 Hz (one gigahertz - one thousand megahertz)

Radio waves

Radio waves - electromagnetic waves with frequencies up to 3 THz, propagating in space. Electromagnetic waves appear in the space around conductors with electric current. Radio waves are mainly used for organizing radio communications, for radiolocation, cooking, and sometimes for medical purposes.

Meter and shorter radio waves are used to communicate with spacecraft units. Cubsat signals are mostly transmitted at the frequencies of 135 MHz and 433 MHz.

We can use an RTL-SDR type radio to receive the signal. It is represented by a software-defined radio (SDR) - a radio station, in which the main part of digital signal processing is done by an ordinary personal computer.

This makes it much cheaper and reduces radio station dimensions. Direct connection with a computer allows users to assess the received signal in a wide frequency band visually, as well as to learn such concepts as frequency, amplification, modulation, etc.

This type of receiver allows users to receive signals in the frequency range from 0.5 MHz to 1750 MHz.

0.5 MHz = 500 kHz = 500,000 Hz

1,750 MHz = 1,750,000 kHz = 1,750,000,000 Hz

RTL.SDR is based on the R820T2 chip, which is an analog signal converter, and RTL2832U chip, which is a digital signal converter completed with the USB interface.

The figure shows the block diagram of the RTL.SDR receiver. First, the R820T2 chip converts the analog signal into an intermediate frequency signal. Second, the intermediate frequency signal is transmitted to the analog-to-digital converter, which converts the incoming analog signal to the digital form (a flow of zeros and ones). The resulting signal is transferred via a USB connector to a computer for subsequent digital processing and output to the monitor. The most complex components of the superheterodyne receiver (filters and demodulators) are implemented in the digital form in the computer, which provides a cheap and more flexible solution.

Introduction to the SDR# software

Let's get familiarize oneselves with capabilities of the SDR# software applied to receive signals from FM radio stations. Run the SDR# software. Control means are located on the left side of the window, and the spectrum of received signals provided in two forms is displayed on the right.

The upper graph shows the instantaneous signal strength plotted along the vertical axis.

At the bottom, the power values are described in terms of brightness and color, and time values are shown on the vertical axis. This graph is called “a waterfall”.

The frequency of received signals is displayed on the horizontal axis. The first time a user launches the program, he/she shall select a corresponding radio type: RTL-SDR connected via USB.

Set the switch to the WFM mode and set the bandwidth to 100000 in the “Radio” section.

Determine the receipt frequency.

If you are in Moscow, you will get a strong signal from the radio station using this frequency.

If you turn up the volume, then you may hear certain radio transmission.

The graph clearly shows that there is another radio station broadcasting using the frequency 103.0 FM. You can change a layout of the graph using the sliders on the right edge of the screen.

If you click on the frequency line and move it to the right or left, you can find other radio stations.

You need to increase the signal strength to receive weak signals. To do this - click on the gear icon.

Move the slider so that the noise level increases by about 10 dB.

Task 1.

Move the frequency configuration line and listen to all Fm radio stations.


A decibel is a relative value which shows how many times one signal is stronger in comparison with another.

1 dB - 1.25 times, 3 dB - 2 times, and 10 dB - 10 times.

When adding the decibels value, a user needs to multiply the time value:

6 dB = 3 dB + 3 dB (2 - 2 = 4 times)

9 dB = 3 dB + 3 dB + 3 dB (2 - 2 - 2 = 8 times)

13 dB = 10 dB + 3dB (10 - 2 = 20 times)

20 dB = 10 dB + 3 dB (10 - 2 = 100 times)

30 dB = 10 dB + 10 dB + 10 dB (10 - 10 - 10 = 1000 times)

Frequency bands.

All radio frequencies are divided into several bands. Some bands are applied for military and special services communications - civilians are not allowed to broadcast in those bands. However, there are also “open” frequency bands in which radio amateurs and civilian organizations - air traffic controllers, railroad workers, power facility workers, etc. are allowed to operate. You will find the following frequency bands, color-coded in SDR#.

Long wave band (LW).

Mid-wave band (MW).

Shortwave band 160m (SW).

Shortwave band 80m (SW).

Shortwave band 40m (SW).

Shortwave band 20m (SW).

Shortwave band 15m (SW).

Civil band 27 MHz. (Citizen`s Band). All radios operate in this range.

Shortwave band 10m (SW).

The frequency range of FM radio stations varies from 87.5 to 108 MHz (VHF)

The frequency range of air traffic controller stations varies from 108 to 137 MHz (VHF)

The amateur frequency range varies from 144 to 148 MHz (VHF)

The military frequency range varies from 225 to 380 MHz (VHF)

The amateur frequency range varies from 430 to 440 MHz (VHF)

Task 2.

Move the frequency line and find all bands in the range from 0 to 500 MHz.

Modulation types.

Modulation is used in order to improve the quality of a radio signal. Modulation is a special way of signal coding. For example, all radio stations which broadcast within the FM band encode the signal in a special way called frequency modulation (FM in English). The next popular modulation method is represented by amplitude modulation (AM in English). A continuous signal is used to transmit Morse code signals. Signals with different modulations appear to be different in the SDR# software.

Frequency modulation.

Typical frequencies: ultra-short waves from 87.5 to 108 MHz

Signal imaging.

Amplitude modulation.

Typical frequencies:

Long waves - from 153 to 279 kHz

Medium waves - from 531 to 279 kHz

Short waves - from 2.3 to 26.1 MHz

Air traffic controllers' conversations - from 108 to 137 MHz

Signal imaging.

As a rule, the SDR# software itself determines the types of radio band modulation characteristics in a correct way. Manual modulation can be activated in the “Radio” menu. FM modulation is called WFM here.

Task 3.

Locate a radio station in the FM band and switch between modulation modes. A normal sound will only be heard using the WFM modulation.

Task 4.

Find air traffic controllers using the range from 108 to 137 MHz. A normal sound will be heard only using the AM modulation.

en/lesson01.txt · Last modified: 2021/04/01 11:50 by golikov

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