The principles of frequency shift keying (FSK) mainly include the following aspects:
Basic principle: Frequency shift keying transmits digital information by changing the frequency of the carrier. In binary frequency shift keying (2FSK), the frequency of the carrier switches between two fixed frequencies f1 and f2 to represent binary data "0" and "1".
Modulation method: The modulation of FSK signals can be achieved in two ways: analog frequency modulation and keying. The phase of the signal generated by the analog frequency modulation method changes continuously between adjacent code elements, which is called continuous phase FSK (CPFSK); while the keying method switches between two independent frequency sources through an electronic switch, and the phase is not necessarily continuous.
Demodulation method: FSK signal demodulation can use incoherent demodulation and coherent demodulation. Incoherent demodulation determines the data by comparing the sampling values of the two signals, while coherent demodulation requires a synchronized local coherent carrier. In addition, there are demodulation methods such as frequency detection, differential detection, and zero-crossing detection.
Application scenario: FSK has good anti-fading ability, so it has been widely used in fading channels. The International Telecommunication Union (ITU) recommends using the 2FSK system when the data transmission rate is lower than 1200b/s.
Advantages and Disadvantages: The advantages of FSK include strong anti-fading ability and simple implementation; the disadvantage is low bandwidth utilization. The frequency band of the phase-discontinuous FSK signal is about three times that of the ASK signal. Therefore, in practical applications, phase-continuous FSK is often used to improve bandwidth utilization.
