Different type of modulation schemes used in Analog Modulation
- 1. What is modulation? Modulation is nothing but, a carrier signal that varies in accordance with the message signal. Modulation technique is used to change the signal characteristics. Basically, the modulation is of following two types: Modulation Analog modulation Amplitude modulation Frequency modulation Phase modulation Digital modulation Amplitude Shift keying Frequency Shift keying Phase Shift keying
- 2. Analog Modulation In analog modulation, analog signal (sinusoidal signal) is used as a carrier signal that modulates the message signal or data signal. The general function Sinusoidal wave’s is shown in the figure below, in which, three parameters can be altered to get modulation – they are amplitude, frequency and phase; so, the types of analog modulation are: 1. Amplitude Modulation(AM) 2. Frequency Modulation(FM) 3. Phase Modulation(PM)
- 3. Amplitude modulation Frequency modulation Phase modulation This type of modulation technique is used in electronic communication. In this modulation, the amplitude of the carrier signal varies in accordance with the message signal, and other factors like phase and frequency remain constant. Its spectrum consists of the lower frequency band, upper frequency band and carrier frequency components. This type of modulation requires more power and greater bandwidth. Amplitude modulation is used in computer modems, VHF aircraft radio, and in portable two-way radio The frequency of the carrier signal varies in accordance with the message signal, and other parameters like amplitude and phase remain constant. Frequency modulation is used in different applications like radar, radio and telemetry, seismic prospecting and monitoring newborns for seizures via EEG, etc. This type of modulation is commonly used for broadcasting music and speech, magnetic tape recording systems, two way radio systems and video transmission systems. The phase of the carrier signal varies in accordance with the message signal. When the phase of the signal is changed, then it affects the frequency. So, for this reason, this modulation is also comes under the frequency modulation. Phase modulation is used for transmitting waves. It is an essential part of many digital transmission coding schemes that underlie a wide range of technologies like GSM, WiFi, and satellite television.
- 5. Analog Modulation Scheme The basic idea here is to superimpose the message signal in analog form on a carrier which is a sinusoid of the form There are three quantities that can be varied in proportion to the modulating signal: the amplitude, the phase, and frequency. The first scheme is called Amplitude Modulation and the second two are called Angle Modulation schemes.
- 6. Amplitude Modulation • In Amplitude Modulation or AM,the carrier signal Acos(w*t) has its amplitude A Modulated in proportion to the message bearing (lower frequency) signal m(t) To give A(1+m(t))cos(wct) The magnitude of m(t) Is chosen to be less than or equal to 1, from reasons having to do with demodulation, i.e. recovery of the signal m(t) from the received signal. The modulation index is then defined to be Figures 1 and 2 are some matlab plots of what the modulated signal looks like for . The frequency of the modulating signal is chosen to be much smaller than that of the carrier signal. Try to think of what would happen if the modulating index were bigger than 1.
- 7. that is transmitted is usually no more than 15 Khz. The bandwidth of an AM scheme which is the amount of space that it occupies in the Fourier domain is twice that of the modulating signal. Thus, for example the radio station KGO 810 occupies roughly the frequency range The version of AM that we described is called Double Side Band AM or DSBAM since we send signals at both wc – wm And wc + wm Note that the AM signal is of the form This frequency components at frequencies Think of how you might demodulate this signal: that is recover the signal from the modulated signal. The AM stations on your radio go from 550 Khz to 1610 Khz, meaning to say that radians per second. The maximum frequency u
- 8. Frequency Modulation FM is a so called angle modulation scheme, it was inspired by phase modulation but has proved to be more useful partly for its ease of generaion and decoding. The main advantages of FM over AM are: 1.Improved signal to noise ratio (about 25dB) w.r.t. to man made interference. 2.Smaller geographical interference between neighboring stations. 3.Less radiated power. 4.Well defined service areas for given transmitter power. Disadvantages of FM: 1.Much more Bandwidth (as much as 20 times as much). 2.More complicated receiver and transmitter. In this scheme the frequency of the modulating signal is changed in proportion to the message signal Thus the signal that is transmitted is of the form Here the signal is assumed to be normalized so that the maximum of the integral is 1 and is called the frequency deviation of the modulation scheme. The index of modulation of an FM signal of the form is defined to be Figures 3, 4, and 5 are examples of what FM signals look like in the time domain for a message signal of the form FREQUENCY MODULATION t
- 9. In general the determination of the frequency content of an FM waveform is complicated, but when is small, for a sinusoidal signal , the bandwidth of the FM signal is On the other hand when is large, the bandwidth is determined (empirically) to be Your FM dial goes from 88 MHz to 108 MHz (this range is in between the frequency ranges for the TV channels 1-6, and 7-12. For a typical radio station, FM modulation with modulating frequency 2, carrier frequency 10 and modulation index 2 FM modulation with modulating frequency 1, carrier frequency 10 and modulation index