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Composition of ultrashort wave communication system

Time：2023-01-04

The ultrashort wave communication system consists of terminal stations and relay stations. The terminal station is equipped with a transmitter, a receiver, a carrier terminal and an antenna. The relay station has only transmitters and receivers with access to both directions, and corresponding antennas.

Transmitter

Indirect FM method is generally used, that is, the method of using phase modulation to obtain FM. In this way, a crystal oscillator with high frequency stability can be used as the main oscillator without the need for a complex frequency control system. However, in order to reduce spurious amplitude modulation and nonlinear distortion, the modulation factor cannot be too large (generally less than 0.5 radian). Therefore, multiple frequency multipliers are used in such transmitters to obtain the desired frequency offset and thereby increase the sideband power at the transmit frequency. The final stage of the transmitter uses a Class C power amplifier with high efficiency. In the ultrashort wave band, lumped parameter elements can be used to form a tuning loop, and microstrip components can be used at the high frequency end.

Receiver

General is a typical FM superheterodyne receiver. It is mainly composed of high frequency amplification, local oscillation, frequency conversion (once or twice), intermediate frequency amplification, limiting, frequency discrimination and baseband amplification. There is more external interference in the ultrashort wave band, so a spiral filter must be added to the input of the receiver and an input band-pass filter must be added to the middle stage to suppress the interference. The frequency modulation signal after the mid-broadcast can be cut off by the limiter to remove the mixed pulse interference or parasitic amplitude modulation wave to improve the signal-to-noise ratio, and then the original baseband signal is restored by the frequency discriminator, amplified, and then output to the user by the carrier terminal.

carrier terminal

The old line baseband signal of the ultrashort wave transmitter and the ultrashort wave receiver is split and combined into multiple two-wire voice signals to connect the user or the equipment connected to the local telephone exchange. Carrier terminals are only installed in VHF terminal stations.

Antenna

Due to the short wavelength of ultrashort wave, a three-unit or five-unit Yagi antenna with simple structure, high gain and good directivity is generally used. Near the high frequency end of the microwave band, an angular reflector antenna can also be used.

Keywords： Composition of ultrashort wave communication system