Frequency Shift Keying (FSK) - Digital To Analog Conversion and Transmission Tutorials Part III

The digital data stream changes the frequency of the carrier signal, fc.
For example, a “1” could be represented by f1=fc +Df, and a “0” could be represented by f2=fc-Df.

Further Reading suggested :

Binary frequency shift keying
Bandwidth of FSK

If the difference between the two frequencies (f1 and f2) is 2Df, then the required BW B will be:

B = (1+d)xS +2Df

Problem Example - 1

We have an available bandwidth of 100 kHz which spans from 200 to 300 kHz. What should be the carrier frequency and the bit rate if we modulated our data by using FSK with d = 1?


This problem is similar to Example 5.3, but we are modulating by using FSK. The midpoint of the band is at 250 kHz. We choose 2Δf to be 50 kHz; this means

Coherent and Non Coherent FSK scheme

In a non-coherent FSK scheme, when we change from one frequency to the other, we do not adhere to the current phase of the signal.

In coherent FSK, the switch from one frequency signal to the other only occurs at the same phase in the signal.

Multi level FSK

Similarly to ASK, FSK can use multiple bits per signal element.
That means we need to provision for multiple frequencies, each one to represent a group of data bits.
The bandwidth for FSK can be higher

B = (1+d)xS + (L-1)/2Df = LxS

Bandwidth of MFSK used in Problem Example-1 

Problem Example-2
We need to send data 3 bits at a time at a bit rate of 3 Mbps. The carrier frequency is 10 MHz. Calculate the number of levels (different frequencies), the baud rate, and the bandwidth.


We can have L = 23 = 8. The baud rate is S = 3 Mbps/3 = 1 Mbaud. This means that the carrier frequencies must be 1 MHz apart (2Δf = 1 MHz). The bandwidth is B = 8 × 1M = 8M. Figure 5.8 shows the allocation of frequencies and bandwidth.


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