1. Principles of FM
FM (Frequency Modulation) means frequency modulation. It is customary to use FM to refer to general FM broadcasting (76-108MHz, 87-108MHz in China, 76-90MHz in Japan), in fact FM is also a modulation method, even in the shortwave range between 27-30MHz, As a band used for amateur radio, space, and satellite communications, there are also frequency modulation (FM) methods. FM radio is FM radio.
Audio signal changes are often cyclical. One of the easiest examples of audio modulation techniques to understand is the violin and string vibrating. The string vibrates rapidly on the strings through fingers and wrists, causing rapid changes in the length of the strings, which ultimately affects the sound of the violin. The softness. Like "FM radio waves", "FM synthesis theory" also has two elements: sound body (carrier) and modulation body. The sound body, or carrier body, is the frequency oscillator that actually emits the sound; the modulator, or modulator, is responsible for adjusting the sound produced by changing the carrier wave. Carrier frequency, modulation frequency and modulation value are important factors that affect FM synthesis theory.
The most basic FMinstrument includes two sinusoidal oscillators, one is a stable carrier frequency fc (CarrierFrequnecy) oscillator; the other is a modulation frequency fm (ModulationFrequency) oscillator. The carrier frequency is added to the output of the modulation oscillator. The carrier oscillator is a simple sine wave frequency with fc frequency. When the modulator occurs, the signal from the modulating oscillator, that is, the sine wave with fm frequency, drives the frequency of the carrier oscillator to change up or down. For example, a 250Hz sine wave modulating wave modulates a 1000Hz sine wave carrier, which means that the 1000Hz frequency generated by the carrier will be subjected to 250 modulations per second. Both the system body and the carrier body are periodic or quasi-periodic oscillators with frequency, amplitude, and waveform.
In frequency modulation technology, the amplitude of the modulation body also plays a key role in frequency modulation. The amplitude of the modulation body affects the depth of change after modulation of the carrier frequency. If the amplitude of the modulation signal is 0, no modulation will occur. Therefore, Like in amplitude modulation (AM), the frequency of the modulating body has an effect on the amplitude of the carrier body, in frequency modulation (FM), the change in frequency of the carrier is also affected by the amplitude of the modulating body.
2. Principle of AM radio:
The AM radio consists of an input loop, a local oscillator loop, a mixing circuit, a detection circuit, an automatic gain control circuit (AGC) and an audio power amplifier circuit, as shown in Figure 1-1.
The input circuit is composed of an antenna coil and a variable capacitor. The local oscillation circuit is composed of a local oscillation coil and a variable capacitor. The local oscillation signal is mixed with the input signal through an internal mixer. The mixed frequency signal is obtained by the intermediate frequency selection loop formed by the intermediate frequency and the 455kHz ceramic filter. At this point, the signal of the radio station has become an amplitude modulated wave with an intermediate frequency of 455kHz as the carrier.
The intermediate frequency signal is amplified by the intermediate frequency, and then the audio signal is obtained by the detection, and the output is amplified by the power, coupled to the speaker, and restored to sound. Among them, the gain of the intermediate amplifier circuit is controlled by the AGC automatic control gain to maintain the automatic adjustment of the gain when the radio signal is different to obtain a consistent listening effect.
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