How does the radio receive signals and make sounds?

The principle of radio is to convert the high-frequency signal received by antenna into audio signal after detection (demodulation), and send it to earphone to become sound wave. With the development of broadcasting, there are many radio waves with different frequencies in the sky. If all these radio waves are received, the audio signal will be like a busy city, with many sounds mixed together and nothing can be heard clearly. In order to try to select the desired program, there is a selection circuit after receiving the antenna. Its function is to select the desired signal (radio station) and "filter out" the unwanted signal to avoid interference. This is the "Select Channel" button that we use when listening to the radio. The output of the selection circuit is the high frequency amplitude modulation signal of a selected radio station. It is not possible to directly push the earphone (electroacoustic device) with it, and it must be restored to the original audio signal. This recovery circuit is called demodulation. When the demodulated audio signal is transmitted to the earphone, the broadcast can be received. As mentioned above, the simplest radio is called a direct detector, but the high-frequency antenna signal obtained from the receiving antenna is generally weak, so it is not appropriate to send it directly to the detector. It is better to insert a high-frequency amplifier between the selection circuit and the detector to amplify the high-frequency signal. Even if a high-frequency amplifier is added, the detected output power is usually only a few milliwatts. It's ok to listen with headphones, but the speaker is too small. Therefore, an audio amplifier is added after the detected output to drive the speaker. High amplifier radio is more sensitive and powerful than direct detection radio, but its selectivity is still poor and its tuning is more complicated. To amplify the high-frequency signal received by the antenna by hundreds or even tens of thousands of times, there are usually several stages of high-frequency amplification, and each stage of circuit has a resonant circuit. When the receiving frequency changes, the resonant circuit must be readjusted, and it is difficult to ensure that the selectivity and passband are exactly the same after each adjustment. In order to overcome these shortcomings, almost all radios now use superheterodyne circuits. The superheterodyne is characterized in that the carrier frequency of the selected high-frequency signal is changed to a lower fixed intermediate frequency (465KHz), and then amplified by an intermediate frequency amplifier to meet the detection requirements before detection. In superheterodyne receiver, in order to generate frequency conversion, there is an additional sine signal, which is usually called heterodyne signal. Circuits that generate heterodyne signals are customarily called local oscillations. There is an intermediate frequency difference between the local oscillator frequency of the radio and the frequency of the received signal, so the mixer and the selection circuit before the local oscillator adopt a unified tuning line, such as using coaxial double capacitors (PVC) for tuning, so as to keep the difference at a fixed intermediate frequency value. Because the intermediate frequency is fixed and the frequency is lower than the high frequency modulation signal, the gain of the intermediate frequency amplifier can be made larger, the work can be more stable, and the passband characteristics can be made ideal, so that the detector can obtain enough signals, so that the whole machine can output audio signals with better sound quality.