Both peak amplitude and frequency remain constant as the phase changes. In phase shift keying, the phase of the carrier is varied to represent two or more different signal elements. If the difference between the two frequencies is 2∆f, then the required bandwidth is We can think of FSK as two ASK signals, each with its own carrier frequency f1 and f2. Again the carrier signals are only simple sine waves, but the modulation creates a non-periodic composite signal with continuous frequencies. The above figure shows the bandwidth of FSK. When the amplitude of NRZ is zero, the oscillator keeps its regular frequency when the amplitude is positive, the frequency is increased. The input to the oscillator is the unipolar NRZ signal. The following figure shows the simplified idea behind the second implementation. Coherent BFSK can be implemented by using one voltage-controlled oscillator (VCO) that changes its frequency according to the input voltage. Non-coherent BFSK can be implemented by treating BFSK as two ASK modulations and using two carrier frequencies. In coherent BFSK, the phase continues through the boundary of two signal elements. In non-coherent BFSK, there may be discontinuity in the phase when one signal element ends and the next begins. There are two implementations of BFSK: non-coherent and coherent. The difference between the two frequencies is 2∆f. Both f1 and f2 are ∆f apart from the midpoint between the two bands. The above figure shows, the middle of one bandwidth is f1 and the middle of the other is f2. We use the first carrier if the data element is 0 we use the second if the data element is 1. In the following Figure, we have selected two carrier frequencies f1 and f2. One way to think about binary FSK (or BFSK) is to consider two carrier frequencies. Both peak amplitude and phase remain constant for all signal elements. The frequency of the modulated signal is constant for the duration of one signal element, but changes for the next signal element if the data element changes. In frequency shift keying, the frequency of the carrier signal is varied to represent data. This is in fact the most important advantage of digital-to- analog conversion. This means if we have a bandpass channel available, we can choose our fc so that the modulated signal occupies that bandwidth. The middle of the bandwidth is where fc the carrier frequency, is located. The most important point here is the location of the bandwidth. The formula shows that the required bandwidth has a minimum value of S and a maximum value of 2S. This means that the bandwidth can be expressed as shown, where S is the signal rate and the B is the bandwidth. However, there is normally another factor involved, called d, which depends on the modulation and filtering process. As we expect, the bandwidth is proportional to the signal rate (baud rate). This signal has a continuous set of frequencies. The carrier signal is only one simple sine wave, but the process of modulation produces a non-periodic composite signal. When the amplitude of the NRZ signal is 1, the amplitude of the carrier frequency is held when the amplitude of the NRZ signal is 0, the amplitude of the carrier frequency is zero. If digital data are presented as a unipolar NRZ digital signal with a high voltage of 1V and a low voltage of 0V, the implementation can achieved by multiplying the NRZ digital signal by the carrier signal coming from an oscillator which is represented in the following figure. The following figure gives a conceptual view of binary ASKS. The peak amplitude of one signal level is 0 the other is the same as the amplitude of the carrier frequency. This is referred to as binary amplitude shift keying or on-off keying (OOK). Both frequency and phase remain constant while the amplitude changes.ĪSK is normally implemented using only two levels. In amplitude shift keying, the amplitude of the carrier signal is varied to create signal elements. This kind of modification is called modulation (shift keying).

Digital information then changes the carrier signal by modifying one or more of its characteristics (amplitude, frequency, or phase). The receiving device is tuned to the frequency of the carrier signal that it expects from the sender. This base signal is called the carrier signal or carrier frequency.

In analog transmission, the sending device produces a high-frequency signal that acts as a base for the information signal. The required bandwidth for analog transmission of digital data is proportional to the signal rate except for FSK, in which the difference between the carrier signals needs to be added.

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