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en:lesson01

Receiving Radio Signals Using RTL-SDR

Frequency

In Hertz, the number of oscillations per second is measured.

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

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

1 GHz = 1,000 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. The main frequencies at which we will receive signals from satellites 135MHz, 433MHz. To receive the signal, we will use a RTL-SDR type radio.

This type of receiver allows you 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

Introducing the SDR # program

Let's get acquainted with the capabilities of the SDR # program by receiving signals from FM radio stations. Run the SDR # program. The controls are located on the left side of the window, and the spectrum of received signals in two forms is displayed on the right.

In the upper graph, the instantaneous signal strength is displayed along the vertical axis.

At the bottom - power is displayed in brightness and color, and the time is plotted on the vertical axis. This graph is due to the characteristic kind of frequencies called waterfall.

The frequency of the received signals is plotted on the horizontal axis.   When you first start the program, you must select the type of radio: RTL-SDR connected via USB.

In the Radio section, set the switch to WFM mode and set the Bandwidth to 100000.

Set the frequency of reception, for example.

If you are in Moscow, then at this frequency there will be a powerful signal from the radio station.

If you now increase the volume, you can hear the radio broadcast.

The graph clearly shows that at a frequency of 103.0 Fm another radio station is broadcasting. The appearance of the graph can be changed using the sliders located on the right border of the screen.

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

In order to receive weak signals, it is necessary to increase the signal gain. To do this, click on the gear.

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


Exercise 1.

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


Decibels.

A decibel is a relative value that shows how many times one signal is more powerful than another.   1 dB - 1.25 times, 3 dB - 2 times, 10 dB - 10 times.

When adding decibels, you need to multiply times:

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 + 3 dB (10 · 2 = 20 times)

20 dB = 10 dB + 10 dB (10 · 10 = 100 times)

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

Frequency ranges.

All radio frequencies are divided into several bands. Some frequency ranges are used for communication by the military and special services - civilians are prohibited from broadcasting in such ranges. But there are also “open” frequency ranges in which radio amateurs and civil organizations are allowed to work - air traffic controllers, railway workers, power engineers, etc. You will find the following frequency ranges marked in color in the SDR #.   Range of long waves (LW).

The range of medium waves (NE).

Amateur shortwave range 160m (HF).

Amateur radio wavelength range 80m (HF).

Amateur shortwave range 40m (HF).

Amateur shortwave range 20m (HF).

Amateur Radio Shortwave Range 15m (HF).

The civilian range is 27MHz. (Citizen`s Band). All walkie-talkies operate in this range.

Amateur shortwave range 10m (HF).

Frequency range of FM radio stations from 87.5 to 108 MHz

Frequency range of air traffic controllers from 108 to 137 MHz

Amateur Radio Band 144-148 MHz

The range of military frequencies is from 225 to 380 MHz.

Amateur radio range from 430 to 440 MHz


Exercise 2.

Move the frequency line and find all ranges from 0 to 500 MHz.


Types of modulation.

In order to improve the quality of the signal transmitted by radio, use modulation. Modulation is a special way to encode a signal. For example, all radio stations that broadcast in the FM band encode a signal in a special way called frequency modulation (English frequency modulation for FM). The next popular modulation method is amplitude modulation (AM amplitude abbreviation). To transmit Morse code signals, CW (Continuous Wave) modulation is used. Signals with different modulations look different in SDR #.

Frequency modulation.

Typical frequencies: ultrashort waves from 87.5 to 108 MHz

The image of the signal.

Amplitude modulation.

Typical frequencies:

Long waves - from 153 to 279 kHz

Medium Waves - 531 to 1.611 kHz

Short waves - from 2.3 to 26.1 MHz

Negotiations of air traffic controllers - from 108 to 137 MHz

The image of the signal.

Typically, the SDR # program itself correctly determines the type of modulation characteristic of the radio wave band. Manual modulation can be switched in the Radio section. FM modulation is called WFM here.


Exercise 3.

Find the radio station in the FM band and switch modulation modes. Normal sound will be heard only with WFM modulation.


Exercise 4.

Find air traffic controller talks in the range of 108 to 137 MHz. Normal sound will be heard only with AM modulation.


en/lesson01.txt · Last modified: 2019/10/21 15:13 by golikov

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