Topic 4: Amplitude Modulation PDF

Topic 4:Amplitude Modulation Knowing : Amplitude Modulation Fundamentals:  1. Carrier Signal A high-frequency signal that carries the information of the message signal. The carrier frequency should be high enough to be transmitted effectively over the airwaves. Typically g...

Topic 4:Amplitude Modulation Knowing : Amplitude Modulation Fundamentals:  1. Carrier Signal A high-frequency signal that carries the information of the message signal. The carrier frequency should be high enough to be transmitted effectively over the airwaves. Typically generated by an oscillator.  2. Message Signal The signal that contains the information to be transmitted (e.g., voice, music, data). This signal usually has a lower frequency compared to the carrier signal.  3. Modulator Combines the carrier signal and the message signal. Types: Balanced Modulator: Produces double sidebands (DSB) and may require further processing to achieve standard AM. Standard AM Modulator: Directly modulates the carrier by varying its amplitude in accordance with the message signal  4. Amplifier Boosts the modulated signal to a level sufficient for transmission. Types: RF Amplifier: Amplifies the radio frequency signal. Power Amplifier: Increases the power level of the signal for effective transmission over longer distances.  5. Antenna Radiates the modulated signal into the air. Should be matched to the frequency of the carrier signal to ensure efficient radiation.  6. Frequency Stability The carrier frequency must be stable to ensure consistent signal transmission and to avoid interference with other channels.  Bandwith and Spectral Efficiency Bandwidth: AM transmission typically requires bandwidth equal to twice the maximum frequency of the message signal. Spectral Efficiency: AM is less efficient in terms of bandwidth usage compared to more modern modulation techniques.  8. Power Efficiency AM transmitters are less power-efficient because a significant amount of power is carried by the carrier, which does not contain information.  9. Signal Quality and Distortion  Signal Quality :Signal quality refers to how well a signal maintains its integrity and how clearly it represents the original information without degradation or loss. Key aspects of signal quality include: ▪ Signal-to-Noise Ratio (SNR): This is a measure of the signal’s strength relative to the background noise. Higher SNR indicates better signal quality and less noise interference. ▪ Signal Strength: Adequate signal strength ensures that the signal can be received and decoded accurately, even over long distances. ▪ Bandwidth: The range of frequencies that the signal occupies. Proper bandwidth allocation is crucial for minimizing distortion and ensuring that the signal fits within the communication channel. ▪ Clarity: This refers to the absence of interference and distortions that can affect the readability or intelligibility of the signal. ▪ Bit Error Rate (BER): In digital communications, BER is the rate at which errors occur in a transmitted signal. Lower BER indicates better signal quality.  Distortion occurs when a signal is altered from its original form due to various factors during transmission or processing. Different types of distortion include: Amplitude Distortion: ❖ Cause: Variations in amplitude response or non-linearities in amplification stages. ❖ Effect: The signal’s amplitude might not be accurately represented, leading to errors in signal interpretation. Frequency Distortion: ❖ Cause: Differences in the frequency response of the system. ❖ Effect: Certain frequencies may be amplified or attenuated more than others, leading to a mismatch between the transmitted and received signals. Phase Distortion: ❖ Cause: Phase shifts introduced by the communication channel or equipment. ❖ Effect: Can cause time dispersion or misalignment of signal components, leading to signal degradation. Intermodulation Distortion: ❖ Cause: Non-linear interactions between different signal components or frequencies. ❖ Effect: Results in the creation of additional unwanted frequencies (intermodulation products) that can interfere with the original signal. Noise: ❖ Types: Includes thermal noise, shot noise, and interferences from other signals. ❖ Effect: Adds unwanted variations to the signal, which can obscure or alter the original message. Multipath Distortion: ❖ Cause: Reflection, diffraction, or scattering of the signal that leads to multiple signal paths. ❖ Effect: Can cause signal fading, echo, or phase shifts, complicating signal reception and decoding. Knowing : Modulation index, carrier Power, Transmitting power modulated and unmodulated current in AM signal transmission  Modulation Index (m): The modulation index (m) is a measure of the extent of modulation in an AM signal. It is defined as the ratio of the amplitude of the modulating signal (message signal) to the amplitude of the carrier signal. 𝐴𝑚 𝑚= 𝐴𝐶 where Am is the peak amplitude of the modulating signal and Ac is the peak amplitude of the carrier signal. The modulation index determines the depth of modulation. An index of 1 corresponds to 100% modulation, while values greater than 1 indicate overmodulation, which can cause distortion.  Carrier Power (P_c): The carrier power is the power of the carrier signal when there is no modulation. It is given by Pc=Ac22RP_c = \frac{A_c^2}{2R}Pc=2RAc2, where AcA_cAc is the peak amplitude of the carrier signal and RRR is the resistance of the load (often the antenna impedance).  Transmitting Power (P_t) for AM Signals: The total power transmitted in an AM signal includes both the carrier power and the power in the sidebands created by modulation. ❑ Unmodulated Current (I_c): The unmodulated current refers to the current flowing in the antenna or transmission line when only the carrier signal is present. It can be calculated using the carrier power and the load resistance ❑ Modulated Current (I_t): The modulated current includes the current due to both the carrier and the modulating signal. When the carrier is modulated, the current can be found using the total transmitted power Summary of Equations/Formula Modulation Index (m) Carrier Power (Pc) Power Transmission :