Frequency Shift Keying (FSK) Working & Applications

The frequency shift keying is the most important digital modulation technique, and it is also known as FSK. A signal has the amplitude, frequency, and phase as properties. Every signal has these three properties. To increase any one of the signal property we can go for the modulation process. Because there are various advantages of the modulation technique. In those some of the advantages are – the antenna size reduced, avoid multiplexing of signals, decrease the SNR, long-range communication can be possible, etc. These are the important advantages of the modulation process. If we modulate the amplitude of the input binary signal according to the carrier signal i.e. called as amplitude shift keying. Here, in this article, we are going to discuss what is frequency shift keying and FSK modulation, demodulation process along with their advantages and disadvantages.

What is Frequency Shift Keying?

It is defined as the changing or improving the frequency characteristics of an input binary signal according to the carrier signal. Amplitude variation is one of the major drawbacks in ASK. So, due to this ask modulation technique used in a few applications only. And its spectrum power efficiency also low. It leads to wastage of power. So to overcome these drawbacks Frequency Shift Keying is preferred. FSK is also known as Binary Frequency Shift Keying (BFSK). The below frequency shift keying theory describes what happening in frequency shift keying modulation.

Frequency Shift Keying Theory

This frequency shift keying theory shows how the frequency characteristics of a binary signal changed according to the carrier signal. In FSK, the binary information can be transmitted through a carrier signal along with frequency changes. The below diagram shows the frequency shift keying block diagram.

fsk-block-diagram
FSK-block-diagram

In FSK, two carrier signals are used to produce FSK modulated waveforms. The reason behind this, FSK modulated signals are represented in terms of two different frequencies. The frequencies are called “mark frequency” and “space-frequency”. Mark frequency has represented logic 1 and space-frequency has represented the logic 0. There is only one difference between these two carrier signals, i.e. carrier input 1 having more frequency than the carrier input 2.

Carrier input 1 = Ac Cos (2ωc+θ) t

Carrier input 2 = Ac Cos (2ωc-θ) t

The switch (s) of the 2:1 multiplexer is having the important role to generate the FSK output. Here the switch is connected to carrier input 1 for all logic 1’s of the binary input sequence. And switch (s) is connected to carrier input 2 for all logic 0’s of the input binary sequence. So, the resultant FSK modulated waveforms have mark frequencies and space frequencies.

fsk-modulation-output-waveforms
FSK-modulation-output-waveforms

Now we will see how the FSK modulated wave can be demodulated at the receiver side. Demodulation is defined as reconstructing the original signal from the modulated signal. This demodulation can be possible in two ways. They are

  • Coherent FSK detection
  • Non-coherent FSK detection

The only difference between the coherent and non-coherent way of detection is the phase of the carrier signal. If the carrier signal we are using at the transmitter side and receiver side are in the same phase while demodulation process i.e. called a coherent way of detection and it is also known as synchronous detection. If the carrier signals which we are using at transmitter and receiver side are not in the same phase then such modulation process known as Non-coherent detection. Another name for this detection is Asynchronous detection.

Coherent FSK Detection

In this synchronous FSK detection, the modulated wave got affected by noise while reaching the receiver. So, this noise can be eliminated from using the bandpass filter (BPF). Here at multiplier stage, the noisy FSK modulated signal is multiplied with the carrier signal from the local oscillator device. Then the resultant signal passes from the BPF. Here this bandpass filter is assigned to cut off frequency which is equal to the binary input signal frequency. So the same frequencies can be allowed to the decision device. Here this decision device gives 0 and 1 for space and mark frequencies of the FSK modulated waveforms.

coherent-fsk-detection
coherent-FSK-detection

Non-coherent FSK Detection

The modulated FSK signal is forwarded from the bandpass filter 1 and 2 with cut off frequencies equals to space and mark frequencies. So, the unwanted signal components can be eliminated from the BPF. And the modified FSK signals are applied as input to the two envelop detectors. This envelope detector is a circuit having a diode (D). Based upon the input to the envelope detector it delivers the output signal. This envelope detector used in the amplitude demodulation process. Based upon its input it generates the signal and then it is forwarded to the threshold device. This threshold device gives the logic 1 and 0 for the different frequencies. This would be equal to the original binary input sequence. So, the FSK generation and detection can be done in this way. This process can be known for the frequency-shift keying modulation and demodulation experiment also. In this FSK experiment, FSK can be generated by the 555 timer IC and detection can be possible by 565IC which is known as a phase-locked loop (PLL).

non-coherent-fsk-detection
non-coherent-FSK-detection

There are few frequency shift keying advantages and disadvantages are listed below.

Advantages

  • Simple process to construct the circuit
  • Zero amplitude variations
  • Supports a high data rate.
  • Low probability of error.
  • High SNR (signal to noise ratio).
  • More noise immunity than the ASK
  • Error-free reception can be possible with FSK
  • Useful in high-frequency radio transmissions
  • Preferable in high-frequency communications
  • Low-speed digital applications

Disadvantages

  •  It requires more bandwidth than the ASK and PSK(phase shift keying)
  • Due to the requirement of large bandwidth, this FSK has limitations to use only in low-speed modems which the bit rate is 1200bits/sec.
  • The bit error rate is less in AEGN channel than phase shift keying.

Thus, the frequency shift keying is one of the fine digital modulation technique to increase the frequency characteristics of the input binary signal. By FSK modulation technique we can achieve error-free communication in a few digital applications. But this FSK has finite data rate and consumes more bandwidth can be overcome by the QAM, which is known as quadrature amplitude modulation. It is the combination of amplitude modulation and phase modulation.



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