The composition of wireless communication system There are many types of wireless communication (or radio communication), which can be classified according to transmission method, frequency range, purpose and so on. Although different wireless communication systems have large differences in equipment composition and complexity, their basic composition remains unchanged. Figure 1-1 is a block diagram of the basic composition of a wireless communication system.
In the figure, the part above the dotted line is the sending device (transmitter), the part below the dotted line is the receiving device (receiver), and the antenna and the antenna switch are shared transceiver devices. The channel is free space. The microphone and the loudspeaker belong to the terminal equipment of communication, which are the source and sink respectively.
Basic composition of wireless communication system
The main feature of the superheterodyne receiver is that the fixed-frequency intermediate frequency amplifier is used to complete the selection and amplification of the received signal; when the signal frequency changes, as long as the local oscillation signal frequency is changed accordingly.
Advantages: Because the intermediate frequency is much lower than the carrier frequency, the Q value of the frequency selection network is much lower, and it is easy to achieve high gain.
Deficiency: Due to the frequency mixing, there is frequency combination interference, and it is more troublesome to suppress the interference.
Digital intermediate frequency structure: The mixed intermediate frequency signal is orthogonally digitized and then digitally demodulated.
Direct transformation structure: adopt zero intermediate frequency, as shown in Figure 1-2.
From the above example, the basic composition of the wireless communication system can be summarized, and it can also be seen that the basic content of the high-frequency circuit should include:
(1) High frequency oscillator: used to generate carrier wave or local oscillation signal
(2) Amplifier: including small signal amplification and power amplification
(3) Mixing or frequency conversion:
(4) Modulation and demodulation:
(5) Feedback control circuit: including AGC, AFC and APC (PLL)
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