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2FSK vs 4FSK | Difference between 2FSK modulation and 4FSK modulation

This page compares 2FSK modulation vs 4FSK modulation and mentions difference between 2FSK modulation and 4FSK modulation types. 2FSK Modulator and 4FSK modulator are used for these modulation types. The links to difference between various modulation types such as QAM, BPSK, QPSK, OQPSK are also mentioned.

FSK stands for Frequency Shift Keying. In this modulation technique, carrier frequency is varied in accordance with the baseband digital inputs while amplitude remains constant. Due to this it is widely employed in modern digital radio systems due to its immunity against noise. It is one of the digital modulation technique in which different frequencies represent different digital binary data.

In the FSK demodulator, data are demodulated from symbols after distinguishing different frequency deviation with respect to the reference carrier frequency used in the FSK modulator and FSK demodulator design.

2FSK Modulator used for 2FSK Modulation

2FSK modulation

Figure-1 depicts 2FSK modulator block. As shown bits or binary digits (0 or 1) are used as inputs while symbols are generated as outputs.

The table-1 below mentions mapping between input and corresponding output used in 2FSK modulation. As mentioned each symbol carries 1 bit of information. 2FSK is same as FSK modulation technique.

Symbol Input Coding
0 carrier-deviation(some offset)
1 carrier+deviation

Mathematically, 2FSK modulation can be expressed using following equations.
S1(t) = Ac*Cos[2*π(Fc+Δf)*t]
S2(t) = Ac*Cos[2*π(Fc-Δf)*t]
Here S1(t) represents output when input is say binary 1
S2(t) represents output when input is say binary 0
Ac is carrier amplitude, Fc is carrier frequency and Δf is frequency deviation as mentioned in the table.

4FSK Modulator used for 4FSK Modulation

4FSK modulation

Figure-2 depicts 4FSK modulator block. As shown dibits (00, 01, 10 or 11) are used as inputs while symbols are generated as outputs.

The table-2 below mentions mapping between input and corresponding output used in 4FSK modulation. As mentioned each symbol carries 2 bits of information. Due to this data rate of 4FSK modulation is double than the data rate of 2FSK modulation.

Symbol input Coding (carrier+Δf value used to map input dibits)
01 carrier-deviation
00 carrier-(1/3)*deviation
10 carrier+(1/3)*deviation
11 carrier+deviation

Similar to the equations mentioned above 4FSK modulation can also be expressed using following equation.
S(t) = Ac*Cos[2*π(Fc+Δf)*t]
Where in 4 different Δf as shown in table-2 are used to represent 4 diferent binary dibits in 4FSK to achieve corresponding outputs.

2FSK power spectrum vs 4FSK power spectrum

The figure-3 depicts 2FSK power spectrum and 4FSK power spectrum. As shown there are 2 carrier peaks in 2FSK modulation while 4 carrier peaks in 4FSK modulation. This is due to the fact that data are represented by 2 carrier phase shifts with respect to the main carrier frequency in the 2FSK while by 4 carrier phase shifts in the 4FSK.

Digital radio technologies DMR (Digital Mobile Radio), dPMR (Digital Private Mobile Radio), LMR and NXDN make use of 4FSK modulation technique.

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