DEVELOPMENT OF THE COMMUNICATION SYSTEM BETWEEN THE CUBESAT SATELLITE AND EARTH

Abstract. According to the magazine “Profile” [1] in 2015, it was running 108 satellites of the Cube Sat format. The satellite standard involves the manufacture of a product in the size of 100 x 100 x 100 mm. The main problem of these satellites is a large radio module for data transmission, comparable to the size of their devices. The solution to this problem can be the development of its own transmitter of small size. In the course of the work it was studied the literature on the topic, created a transmitting device, mastered the program of reception of radio signals. Also, there was a difficulty with the creation of a high-frequency generator, which was solved by using a surfactant resonator in conjunction with a high-frequency transistor. The transmitter was tested for 72 hours of continuous operation; the transmitter with different supply voltage was tested. The dimensions of the created transmitter were 20 x 12 mm. This transmitter was already presented at the conference within the framework of the program “Satellite of my school”, held on June 12, 2018 in Kurganinsk, and at the scientific and technical festival “From the screw!”, held on September 14-16, 2018 in Krasnodar. In the future, it is planned to develop feedback and integrate the system into cubesat satellites.

Keywords: satellite, radio transmitter, antenna, generator, communication.

The complexity of manufacturing and configuring the elements and components of the transmitter depends on the frequency. On the one hand, the higher the frequency is more difficult to manufacture and higher cost. On the other hand, the higher the frequency of the generator, the less energy will be spent on transmission and smaller antennas will be required. In turn, the frequency deviation affects the coordinated operation of the transmitter and receiver. For example, a 1% deviation in the frequency of a medium-wave (300 kHz) transmitter will cause a ±3 kHz frequency change, which is acceptable. A deviation of 1% of the transmitter operating at a frequency of 450 MHz, will give a frequency deviation of ±4.5 MHz.

Modulation is the process of combining a low-frequency (LF) signal with a reference oscillator frequency. Modulation in a certain way changes the form of high-frequency (HF) oscillations and there are several types (figure 1). In radio most often use amplitude (AM) and frequency modulation (FM).

Illustration 1. Signal modulation

The modulating signal changes either the amplitude of the carrier or its frequency. In any case, the carrier is loaded with a useful signal.

The main components of the transmitter are the RF generator and modulator. If necessary, add a bass amplifier (input signal) and an RF amplifier (output signal) (figure 2).

Illustration 2. The simplest circuit of the transmitter

To create a reliable transmitter, it was decided to use a resonator on surface acoustic waves (surfactants). Surfactant resonators are widely used in highly stable generators, band pass filters and sensors of physical quantities. The design of a single-pass resonator on the surfactant includes a counter-screw Converter located on the surface of the piezoelectric medium, to the right and left of which there are reflective structures. The basic piezoelectric material for resonators on surfactants serves as highly stable cuts of quartz. However, other piezoelectric materials, such as lithium niobate and tantalite, are also used in surfactant filters when resonators are used.

Electric high-frequency signal through transducers creates mechanical (acoustic) vibrations on the surface of quartz, propagating in the form of a wave. This wave is called - surface acoustic wave. The surfactant velocity in quartz is 100,000 times less than the electromagnetic wave velocity. The slow propagation of the acoustic wave is the basis of miniaturization of surfactants. The maximum conversion efficiency is achieved at the frequency of synchronism, that is, at such a frequency of the input electrical signal, when the wavelength of the acoustic oscillations coincides with the spatial period of the transducer electrodes. At the frequency of 433.92 MHz, the wavelength of acoustic oscillations is 7 microns.

In any transmitter is required to present a reference oscillator. Consider the scheme of the reference generator on the saw resonator R433, operating at a frequency of 433.92 MHz (figure 3).

Illustration 3. Reference oscillator circuit

The current from the surfactant of the resonator controls the transistor Q1. The C1 capacitor is used to filter the constant component. If you connect an antenna to the output of this generator, when listening to the frequency of 433.92 MHz, we hear silence. So you can check the performance of the generator. The signal of this generator can be modulated by means of a bipolar transistor interrupting the power supply (figure 4).

Illustration 4. Transmitter circuit without RF amplifier

In this scheme, the transistor Q2 is modulating. The power connection to the R2 resistor keeps the transistor always open. The C3 capacitor is designed to protect the signal source from the constant component. Adding to the circuit transistor Q3 can increase the power to 1.3 W (figures 5, 6).

Figure 5. The scheme of the transmitter with the output amplifier stage

Illustration 6. Photo of the finished transmitter

The transmitter was tested without the output amplifier stage with antenna type: asymmetric vibrator – length 17.3 cm.

RTL-SDR with Rafael Micro R820T chip was used as a receiver. It was connected to the antenna “symmetrical vibrator”. Length one shoulder 13,3 cm Signal can be recognized at a distance of 6 km from the transmitter. The output power of the transmitter without the amplifier stage was 12 mW at a supply voltage of 5 V. After increasing the supply voltage to 12 V, the range increased to 10 km, and the power became equal to 24 mW. The transmitter was also tested continuously at 12 V for 72 hours. After testing, the transmitter functioned normally.

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