Frequency Shift Keying Demodulator Working (FSK):

In digital data communication, binary data is transmitted by means of a carrier frequency. It uses two different carrier frequencies for logic 1 and logic 0 states of binary data signal. This type of data transmission is called Frequency Shift Keying (FSK). In this data transmission, on the receiving end, two carrier frequencies are converted into 1 and 0 to get the original binary data. This process is called as Frequency Shift Keying Demodulator.

A PLL can be used as a Frequency Shift Keying Demodulator, as shown in the Fig. 2.132.

It is similar to the PLL demodulator for analog FM signals except for the addition of a comparator to produce a reconstructed digital output signal.

Let us consider that there are two frequencies, one frequency (f₁) is represented as “0” and other frequency (f₂) is represented as “1”. If the PLL remain is locked into the FSK signal at both f₁ and f₂; the VCO control voltage which is also supplied to the comparator will be given as

V_C1 = (f₁ – f₀) / K_v and

V_C2 = (f₂ – f₀) / K_v , respectively.

where

• K_v is the voltage to frequency transfer coefficient of the VCO.

The difference between the two control voltage levels will be

Δ V_C = (f₂ – f₁) / K_v.

The reference voltage for the comparator is derived from the additional low pass filter and it is adjusted midway between V_C1 and V_C2.

Therefore, for V_C1 and V_C2, comparator gives output ‘0’ and ‘1’, respectively.