Amplitude modulation and frequency modulation
- 1. Amplitude Modulation Amplitude Modulation communication Electronics communication Electronics By M Z khan By M Z khan
- 2. Introduction Introduction Amplitude Modulation is the simplest and earliest form of transmitters AM applications include broadcasting in medium- and high-frequency applications, CB radio, and aircraft communications
- 3. Basic Amplitude Modulation Basic Amplitude Modulation The information signal varies the instantaneous amplitude of the carrier
- 4. AM Characteristics AM Characteristics AM is a nonlinear process Sum and difference frequencies are created that carry the information
- 5. Full-Carrier AM: Time Full-Carrier AM: Time Domain Domain Modulation Index - The ratio between the amplitudes between the amplitudes of the modulating signal and carrier, expressed by the equation: c m E E m =
- 6. Overmodulation Overmodulation When the modulation index is greater than 1, overmodulation is present
- 7. Modulation Index for Multiple Modulation Index for Multiple Modulating Frequencies Modulating Frequencies Two or more sine waves of different, uncorrelated frequencies modulating a single carrier is calculated by the equation: m m1 2 m2 2
- 9. Full-Carrier AM: Frequency Domain Full-Carrier AM: Frequency Domain Time domain information can be obtained using an oscilloscope Frequency domain information can be calculated using Fourier methods, but trigonometric methods are simpler and valid Sidebands are calculated using the formulas at the right fusb fc fm flsb fc fm Elsb Eusb mEc 2
- 10. Bandwidth Bandwidth Signal bandwidth is an important characteristic of any modulation scheme In general, a narrow bandwidth is desirable Bandwidth is calculated by: m F B 2
- 11. Power Relationships Power Relationships Power in a transmitter is important, but the most important power measurement is that of the portion that transmits the information AM carriers remain unchanged with modulation and therefore are wasteful Power in an AM transmitter is calculated according to the formula at the right Pt Pc 1 m2 2
- 12. Quadrature AM and AM Stereo Quadrature AM and AM Stereo Two carriers generated at the same frequency but 90º out of phase with each other allow transmission of two separate signals This approach is known as Quadrature AM (QUAM or QAM) Recovery of the two signals is accomplished by synchronous detection by two balanced modulators
- 14. Suppressed-Carrier AM Suppressed-Carrier AM Full-carrier AM is simple but not efficient Removing the carrier before power amplification allows full transmitter power to be applied to the sidebands Removing the carrier from a fully modulated AM systems results in a double-sideband suppressed-carrier transmission
- 16. Single-Sideband AM Single-Sideband AM The two sidebands of an AM signal are mirror images of one another As a result, one of the sidebands is redundant Using single-sideband suppressed-carrier transmission results in reduced bandwidth and therefore twice as many signals may be transmitted in the same spectrum allotment Typically, a 3dB improvement in signal-to- noise ratio is achieved as a result of SSBSC
- 17. DSBSC and SSB DSBSC and SSB Transmission Transmission