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FSK (Frequency-shift keying) is a modulation method used earlier in information transmission. Its main advantages are: it is easier to implement, and it has better performance against noise and attenuation. It has been widely used in low and medium speed data transmission. The most common is a dual-frequency FSK system that uses two frequencies to carry binary ones and zeros.

Introduction of FSK

Technical FSK has two categories, incoherent and coherent FSK. In non-coherent FSK, the transfer between instantaneous frequencies is named by two discrete values named mark and spatial frequency. On the other hand, in coherent frequency shift keying or binary FSK, there is no break in the output signal.

In the digital era, computer communications are transmitted on data lines (telephone lines, network cables, optical fibers, or wireless media) using FSK-modulated signals, which convert binary data into FSK signals for transmission, which in turn will receive the The FSK signal is demodulated into binary data and converted into a binary language represented by high and low levels, which is a language that the computer can directly recognize.

FSK modulation

In binary frequency shift keying, the frequency of a carrier signal with constant amplitude is switched with the change of the input code stream (called treble and bass, representing binary 1s and 0s).

The easiest way to generate the FSK signal is to switch between two independent oscillators based on whether the input data bit is 0 or 1. The waveform generated by this method has a discontinuous phase at the moment of switching, so this FSK signal is called a discontinuous FSK signal.

Due to the discontinuity of the phase will cause spectrum spreading, this FSK modulation method is often used in traditional communication equipment. With the continuous development of digital processing technology, successive phase FSK modulation technology is increasingly used.

At present, the more commonly used method for generating FSK signals is to first generate FSK baseband signals and use the baseband signals to frequency modulate a single carrier oscillator.

The power spectral density function of the FSK signal with continuous phase eventually fades according to the negative fourth power of the frequency offset. If the phase is discontinuous, the power spectral density function fades according to the negative second power of the frequency offset.

In the comprehensive experimental system of communication principles, the modulation scheme of FSK is as follows:

FSK signal: S(t)=cos(ω0t+2πfi·t)

In the baseband signal of the FSK mode of the communication channel, fH frequency is used for transmission signals, and fL frequency is used for empty signals. In FSK mode, the Hamming error correction coding and decoding technology is not used. The data sources provided by the modulator are:

1. External data input: can come from synchronous data interface, asynchronous data interface and m sequence;

2. All 1 code: can test the transmission frequency (high) when the number is transmitted;

3. All 0 codes: can test the sending frequency when the number is empty (low);

4. Code 0/1: 0101. . Alternate pattern, used for general testing;

5. Special code sequence: a code sequence with a period of 7 to facilitate observation by a conventional oscilloscope;

6, m sequence: used to test the performance of the channel;

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