 Hello everyone. Welcome to you all for this session. I am Dr. Asha Tharangi and today we are going to discuss another digital modulation technique FSK that is frequency shift keying, its modulation and demodulation techniques. At the end of this session students will be able to represent given bit stream into FSK signal and explain FSK modulation and demodulation techniques. These are the contents that will be covered in this session. In the previous session we discussed about digital modulation techniques of amplitude shift keying. We have seen that in amplitude shift keying that is in binary ASK the amplitude of the carrier is varied with respect to one or zero of the input bit stream. Frequency shift keying popularly known as FSK is another digital modulation technique in which frequency of carrier signal is varied with respect to input bit one or zero of digital data keeping the frequency and phase constant. Thus in binary FSK the signal is represented by two different sinusoidal carrier having different frequencies but same amplitude and phase. Figure shows an example of a bit stream and its FSK signal representation. It can be seen that when the input bit is one FSK is represented by high frequency sinusoidal signal and when input bit is zero it is represented by a low frequency sinusoidal signal having both amplitude and phase constant. Thus in mathematical form FSK signal is represented as V FSK of t is equals to a cos omega c1 t when input bit is one and a cos omega c2 t when input bit is zero. Omega c1 and omega c2 are the two different angular frequencies for two respective carriers. Let us now see how FSK signal is generated. A simple logic to generate a binary FSK signal is to use a 2 is to 1 multiplexer. To the one input of this multiplexer high frequency sinusoidal signal is applied. Say V1 of t is equals to a cos omega c plus ohm t and to the another input a low frequency sinusoidal signal is applied say V2 of t is equals to a cos omega c minus ohm t. These two signals have same amplitude and phase but differ only in frequency. These two frequencies can be chosen with respect to some reference frequency say omega c. The input bit stream is applied to the select pin of this multiplexer. Thus the rate of switching of the switch is same as the bit rate say RB. The output of this multiplexer is simply BFSK signal as shown in the figure. When the input is logic one the switch connects input one to output thus giving a higher frequency carrier signal V1 of t at the output during the bit slot. And when the input bit is zero switch connects input two to the output thus giving a low frequency carrier V2 of t at the output during its time slot. Thus the multiplexed output is a BFSK signal. Now before moving ahead pause the video and recall ASK modulator used to generate the OOK signal. Welcome back. Figure shows the product modulator used to generate OOK signal. To the one input bit stream in NRZ format is applied and to the another input carrier signal is applied and the output is simply the product of these two signals resulting into OOK signal output. This same ASK modulator is used to generate the FSK signal. Let us now see how FSK signal is generated using ASK modulator. Figure shows BFSK modulator using ASK modulators. In this two ASK modulators are used. Input bit stream in NRZ format is applied directly to ASK modulator one. Whereas the inverted bit stream is applied to the ASK modulator two. To the other inputs of ASK modulators different frequency sinusoidal carrier signals say V1 of t A cos omega c plus ohm t and V2 of t A cos omega c minus ohm t are applied as shown. The output of ASK modulator one is as shown. It is the product of input bit stream and the carrier V1 of t. Similarly the output of ASK modulator two is a product of inverted bit stream and the carrier V2 of t. These two outputs of ASK modulators are further applied to the summing circuit whose output is a BFSK signal. When the input bit is one output of ASK modulator one is a high frequency carrier V1 of t. While its respective inverted value zero makes the output of ASK modulator two zero or no signal during that bit slot. The resultant output of the summing device is an addition of these two outputs and is V1 of t that is A cos omega c plus ohm t during the bit slot. Similarly when the input bit is zero its inverted output is one. Resulting in a no signal at ASK modulator one output and a low frequency signal V2 of t at ASK modulator two output which are then applied to the summing device. Thus during this time slot the resultant FSK output is a low frequency V2 of t that is A cos omega c minus ohm t. In this way the FSK signal is generated using ASK modulators. Let us now see how FSK signal is demodulated. Figure shows block diagram for the coherent FSK demodulation technique. It consists of two synchronous detectors, difference amplifier, low pass filter and a comparator. The incoming FSK signal is applied to both the synchronous detectors. Another input to synchronous detectors are the reference signals cos omega c plus ohm t and cos omega c minus ohm t having the same frequency and phase as that of the carrier signals used for FSK modulation. The output of both the synchronous detectors are applied to the difference amplifier whose output is then fed to the low pass filter and comparator. Let us consider the incoming FSK signal during the current bit slot is A cos omega c plus ohm t. The output of synchronous detector one is a product of A cos omega c plus ohm t and cos of omega c plus ohm t which results into a sinusoidal signal expressed as shown. Similarly, the output of synchronous detector two is a product of A cos omega c plus ohm t and cos of omega c minus ohm t which results into a sinusoidal signal expressed as shown. These two products are further applied to the difference amplifier. Which results into a signal consisting of a positive DC term A by 2 along with some frequency components. This signal is then applied to a low pass filter whose output is only DC component A by 2 which is further fed to a comparator having a reference voltage set to zero volts. As input voltage A by 2 is greater than reference comparator output is logic one. Similarly, during the bit slot when FSK signal is A cos omega c minus ohm t low pass filter output produces negative DC component minus A by 2 which when applied to a comparator gives the output logic zero. In this way by using synchronous detectors FSK signal is demodulated. Let us now see non-coherent FSK demodulated modulation techniques. Figure shows the block diagram of non-coherent FSK demodulator. It consists of a band pass filter, envelope detector, summing circuit and threshold device. The two band pass filters are designed to have a pass band frequency same as those frequencies used for FSK modulation. During each bit slot the incoming FSK signal with some noise is applied to both the band pass filters. Let us assume that during the current bit slot the incoming FSK signal is high frequency carrier A cos 2 pi f c plus f d t. This signal is passes through band pass filter 1 and appears at its output. But the band pass filter 2 with pass band frequency f c minus f d blocks it thus giving no signal at its output. The envelope detector 1 produces a positive voltage at its output. Whereas the output of envelope detector 2 is zero. These outputs of both the envelope detectors are fed to a summing circuit which gives a resultant positive voltage. This voltage is then compared in a threshold device having zero volt as a reference value. The demodulated output is thus detected as logic 1. Again consider a bit slot when incoming FSK signal has frequency f c minus f d. This signal now appears at the output of band pass filter 2 whereas no signal is present at the output of band pass filter 1. Thus resulting in a positive voltage at the output of envelope detector 2 and zero volt at the output of envelope detector 1. This gives a negative voltage at the output of summing circuit. At this negative voltage is less than the reference voltage the threshold device detects its as a logic zero. In this way during each bit slot FSK signal is demodulated as bit 1 is or zero. As nowhere in the circuit additional reference synchronizing signal is used this is a non coherent type of demodulation technique. So in this session we have covered what is FSK signal, its representation and its modulation and demodulation technique. These are the list of references you can use. Thank you.