Frequency Shift Keying-FSK
Table of Contents
The digital modulation method known as Frequency Shift Keying (FSK) alters the carrier signal’s frequency in response to changes in the digital signal. FSK is a frequency modulation scheme. So For a binary High input, the output of an FSK modulated wave has a high frequency, and for a binary Low input, it has a low frequency. The Mark and Space frequencies refer to the binary 1s and 0s.
The diagrammatic representation of the FSK modulated waveform and its input is shown in the image below.
Let’s learn how an FSK modulator functions in order to determine the procedure for obtaining this FSK modulated wave.
Frequency Shift Keying Modulator
The two oscillators, a clock, and the input binary sequence are shown in the block diagram of an FSK modulator. Here is a block diagram of it.
A switch, an internal clock, and two oscillators that produce higher and lower frequency signals are connected to the oscillators. A clock is applied internally to both oscillators to prevent the output waveform from abruptly changing phases while the message is being transmitted. So The transmitter uses the binary input sequence to select the frequencies in accordance with the binary input.
A FSK wave can be demodulated in a variety of ways. Asynchronous and synchronous detectors are the two primary strategies for FSK detection. Coherent detectors include synchronous detectors, whereas incoherent detectors include asynchronous detectors.
Asynchronous FSK Detector
Two band pass filters, two envelope detectors, and a decision circuit make up the Asynchronous FSK detector’s block diagram. The diagrammatic representation is shown below.
The two Band Pass Filters BPFs, tuned to the Space and Mark frequencies, pass the FSK signal through them. These two BPFs’ outputs resemble an ASK signal, which is fed to the envelope detector. Each envelope detector’s signal is asynchronously modulated. The decision circuit selects the most probable output from among all of the envelope detectors. Additionally, a rectangular waveform is created.
Synchronous FSK Detector
So Two mixers with built-in local oscillator circuits, two band pass filters, and a decision circuit make up the Synchronous FSK detector’s block diagram. The diagrammatic representation is shown below.
The two mixers with built-in local oscillator circuits receive the input of the FSK signal. Two band pass filters are connected to these two. So The decision circuit determines which output is more likely and selects it from any one of the detectors using these combinations as demodulators. There is a minimum frequency difference between the two signals. So The bandwidth of each demodulator for the two is a function of its bit rate. Compared to asynchronous type demodulators, this synchronous demodulator is a little more complex.