[Radio Principle]Schematic diagram of the radio and analysis of the main principle of the radio
The main principle of the working principle of the radio
Radio broadcasting system, including radio and television systems. They are the signals to be transmitted and wirelessly transmitted.
The frequency of the audio signal is 0-20KHz. And these lower frequency signals cannot be transmitted and transmitted directly by radio. Therefore, it must be facilitated by higher frequency radio frequency signals suitable for transmission and long-distance propagation. These high-frequency signals are called carrier frequency signals or carrier signals.
There are two main ways to add audio signal or video signal to carrier frequency signal, AM and FM.
The radio that uses the former signal processing method is called AM radio, and the radio is called AM radio. The frequency range of its radio waves is mainly concentrated in two frequency bands; mid-band 535KHz-1605KHz, short-band 1.6MHz-26MHz.
Broadcasts that use the latter method of processing signals are called FM radios, and radios are called FM radios. The frequency range of its radio waves is mainly concentrated in the ultra-short band 87MHz-108MHz.
In the wireless TV broadcasting system, both methods of signal processing are used. The audio signal adopts the frequency modulation method, and the full TV signal adopts the amplitude modulation method.
As for the working principle, there are mainly two types, high-amplification type and super-heterodyne type.
The high-amp type, also known as the direct-release type and the regenerative duplex type, is an early model with poor performance. Now it is generally only used for teaching, testing or occasions with low requirements.
The radios and televisions on the market are almost without exception superheterodyne receivers with excellent performance.
RadioMain principle analysis
The principle of the radio is to detect (demodulate) the high-frequency signal received from the antenna. ) is restored to an audio signal, and sent to the earphone to become a sound wave. Due to the development of broadcasting, there are many radio waves of different frequencies in the sky. If all these many radio waves are received, the audio signal will be like in a busy city, and many sounds will be mixed together, and as a result, nothing can be heard. In order to try to select the desired program, after the receiving antenna, there is a selective circuit, its function is to select the desired signal (station) and “filter out” the unwanted signal to avoid interference, which is When we listen to the radio, we use the “Select Channel” button.
The output of the selective circuit is to select the high-frequency AM signal of a certain radio station. It is not acceptable to use it to directly push the earphone (electroacoustic device), and it must be restored to the original audio signal. This kind of restoration circuit is called demodulation, and the demodulated audio signal is sent to the earphone, and the broadcast can be received.
The above is the simplest radio called direct detector, but the high-frequency antenna electrical signal obtained from the receiving antenna is generally very weak, and it is not suitable to send it directly to the detector. A high-frequency amplifier is inserted between the selection circuit and the detector to amplify the high-frequency signal. Even if a high-frequency amplifier has been added, the power of the detection output is usually only a few milliwatts, which can be heard with headphones, but it is too small to use a speaker, so an audio amplifier is added after the detection output to push the speaker.
High-amplifier radios are more sensitive and powerful than direct-detection radios, but have poor selectivity and complex tuning. Amplify the high-frequency signal received from the antenna by hundreds or even tens of thousands of times. Generally, several stages of high-frequency amplification are required. Each stage of the circuit has a resonant circuit. When the received frequency changes, the resonant circuit has to be regenerated. In order to overcome these shortcomings, almost all modern radios use super-heterodyne circuits.
The characteristics of superheterodyne are: the carrier frequency of the selected high-frequency signal becomes a lower fixed intermediate frequency (465KHz), and then the intermediate frequency amplifier is used to amplify to meet the requirements of detection, and then detection is performed. In the superheterodyne receiver, in order to generate frequency conversion, there is also an external sinusoidal signal, which is usually called a heterodyne signal, and the circuit that generates the heterodyne signal is used to be called local oscillation. The frequency of the local oscillator of the radio is different from the frequency of the received signal by an intermediate frequency, so the selection circuit before the mixer and the local oscillator use a unified tuning line, such as using a coaxial double capacitor (PVC) to tune, so that the difference is Keep a fixed IF value. Since the intermediate frequency is fixed and the frequency is lower than that of the high-frequency modulated signal, the gain of the intermediate amplifier can be made larger, the work is relatively stable, and the passband characteristics can also be ideal, so that the detector can obtain a large enough signal. So that the whole machine can output the audio signal with better sound quality.
The broadcast program of a radio station is to first convert the sound into an audio electrical signal through a microphone, and after amplification, it is modulated by a high-frequency signal (carrier). At this time, a certain parameter of the high-frequency carrier signal follows the audio signal. Make corresponding changes, so that the audio signal we want to transmit is included in the high-frequency carrier signal, the high-frequency signal is amplified, and then the high-frequency current flows through the antenna to form radio waves that are emitted outward, and the radio wave propagation speed is 3 ×108m/s, this radio wave is received by the radio antenna, then amplified and demodulated, restored to an audio electrical signal, sent into the speaker voice coil, causing the corresponding vibration of the paper cone, and the sound can be restored, that is, sound and electricity Conversion Transmission – The process of electro-acoustic conversion.
The frequency of medium wave (high frequency carrier frequency) is specified as 525-1605kHz (thousand cycles).
The frequency range of short wave is 3500-18000kHz.
