FSK (Frequency Shift Keying)

Frequency Shift Keying (FSK) is a digital modulation technique used for transmitting digital data over an analog carrier signal. It is a form of frequency modulation (FM) where the modulating signal consists of a series of binary digits (0's and 1's) that are used to shift the carrier frequency between two predefined values. FSK is commonly used in a variety of applications, including data communication, telemetry, and remote control.

FSK is based on the concept of binary phase shift keying (BPSK), which is a simpler form of digital modulation. In BPSK, the carrier frequency is kept constant, and the phase of the signal is shifted by 180 degrees to represent a binary 1 or 0. However, this method is not suitable for high-speed data transmission since it requires a wide bandwidth to accommodate the rapid phase changes.

FSK, on the other hand, uses two frequencies to represent the binary 1 and 0. A logic 1 is transmitted by using a carrier frequency f1, while a logic 0 is transmitted by using a carrier frequency f2. The two frequencies are typically chosen to be close to each other, with the difference between them referred to as the frequency shift or deviation.

The FSK signal can be expressed as:

s(t) = A * sin(2πf1t) if the input is a logic 1

s(t) = A * sin(2πf2t) if the input is a logic 0

where A is the amplitude of the carrier signal, and t is the time.

In FSK, the modulation index is defined as the ratio of the frequency shift to the bit rate. The modulation index determines the bandwidth required to transmit the signal and also affects the signal-to-noise ratio (SNR). If the modulation index is too high, the signal will occupy a wider bandwidth, and the SNR will be reduced. On the other hand, if the modulation index is too low, the signal may not be distinguishable from noise.

There are two types of FSK modulation: coherent and non-coherent. In coherent FSK, the receiver has knowledge of the carrier frequency and phase, and the demodulation process involves mixing the received signal with a local oscillator signal that is phase-locked to the carrier. In non-coherent FSK, the receiver does not have knowledge of the carrier frequency and phase, and the demodulation process involves envelope detection of the received signal.

One of the advantages of FSK is its immunity to amplitude variations, which makes it more robust against noise and interference compared to amplitude modulation (AM) and phase modulation (PM). FSK is also relatively simple to implement and can be used with a variety of carrier frequencies and modulation indices.

However, FSK has some limitations, including its sensitivity to frequency offset and phase noise, which can cause errors in demodulation. It also requires a wider bandwidth compared to other digital modulation techniques, which may be a limiting factor in some applications.

In summary, FSK is a digital modulation technique that uses two carrier frequencies to represent binary 1 and 0. FSK is relatively simple to implement and is robust against amplitude variations, but it has some limitations, including sensitivity to frequency offset and phase noise and a wider bandwidth requirement compared to other digital modulation techniques.